fix unit tests

### Bug Description

The unit test are broken:

https://github.com/canonical/cos-proxy-operator/actions/runs/24735374187/job/72360682142?pr=230

### To Reproduce

1. `tox -e unit`

### Environment

main branc

### Relevant log output

```shell
=================================== FAILURES ===================================
_ COSProxyCharmTest.test_alert_rules_are_forwarded_on_adding_prometheus_then_targets _
Traceback (most recent call last):
File "/usr/lib/python3.12/unittest/case.py", line 58, in testPartExecutor
yield
File "/usr/lib/python3.12/unittest/case.py", line 634, in run
self._callTestMethod(testMethod)
File "/usr/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
if method() is not None:
^^^^^^^^
File "/usr/lib/python3.12/unittest/mock.py", line 1390, in patched
return func(*newargs, **newkeywargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_charm.py", line 434, in test_alert_rules_are_forwarded_on_adding_prometheus_then_targets
self.assertCountEqual(groups, GENERIC_RULES + BUNDLED_RULES + expected_groups)
File "/usr/lib/python3.12/unittest/case.py", line 1216, in assertCountEqual
self.fail(msg)
File "/usr/lib/python3.12/unittest/case.py", line 715, in fail
raise self.failureException(msg)
AssertionError: Element counts were not equal:

Diff is 3712 characters long. Set self.maxDiff to None to see it.
------------------------------ Captured log call -------------------------------
DEBUG root:storage.py:68 Initializing SQLite local storage: :memory:.
DEBUG cosl.cos_tool:cos_tool.py:191 Could not locate cos-tool at: "cos-tool-amd64"
DEBUG cosl.cos_tool:cos_tool.py:62 Skipping injection of juju topology as label matchers
DEBUG cosl.cos_tool:cos_tool.py:155 `cos-tool` unavailable. Leaving expression unchanged: vector_uptime_seconds < (vector_uptime_seconds offset 5m)
DEBUG cosl.rules:rules.py:350 Reading rule from src/prometheus_alert_rules/vector_restarted.rule
DEBUG cosl.cos_tool:cos_tool.py:155 `cos-tool` unavailable. Leaving expression unchanged: up < 1
DEBUG cosl.cos_tool:cos_tool.py:155 `cos-tool` unavailable. Leaving expression unchanged: absent(up)
_ COSProxyCharmTest.test_alert_rules_are_forwarded_on_adding_targets_then_prometheus _
Traceback (most recent call last):
File "/usr/lib/python3.12/unittest/case.py", line 58, in testPartExecutor
yield
File "/usr/lib/python3.12/unittest/case.py", line 634, in run
self._callTestMethod(testMethod)
File "/usr/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
if method() is not None:
^^^^^^^^
File "/usr/lib/python3.12/unittest/mock.py", line 1390, in patched
return func(*newargs, **newkeywargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_charm.py", line 485, in test_alert_rules_are_forwarded_on_adding_targets_then_prometheus
self.assertCountEqual(groups, GENERIC_RULES + BUNDLED_RULES + expected_groups)
File "/usr/lib/python3.12/unittest/case.py", line 1216, in assertCountEqual
self.fail(msg)
File "/usr/lib/python3.12/unittest/case.py", line 715, in fail
raise self.failureException(msg)
AssertionError: Element counts were not equal:

Diff is 3712 characters long. Set self.maxDiff to None to see it.
------------------------------ Captured log call -------------------------------
DEBUG root:storage.py:68 Initializing SQLite local storage: :memory:.
/home/runner/work/cos-proxy-operator/cos-proxy-operator/src/vector.py:44: SyntaxWarning: invalid escape sequence '\d'
DEFAULT_VECTOR_CONFIG = """

tests/unit/test_charm.py: 17 warnings
/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_charm.py:235: PendingDeprecationWarning: Harness is deprecated. For the recommended approach, see: https://documentation.ubuntu.com/ops/latest/howto/write-unit-tests-for-a-charm/
self.harness = Harness(COSProxyCharm)

tests/unit/test_charm.py: 1 warning
tests/unit/test_outgoing_relation_data.py: 14 warnings
/home/runner/work/cos-proxy-operator/cos-proxy-operator/lib/charms/grafana_agent/v0/cos_agent.py:690: PydanticDeprecatedSince20: The `json` method is deprecated; use `model_dump_json` instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.13/migration/
relation.data[self._charm.unit][data.KEY] = data.json()

tests/unit/test_endpoint_aggregator.py: 12 warnings
/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_endpoint_aggregator.py:135: PendingDeprecationWarning: Harness is deprecated. For the recommended approach, see: https://documentation.ubuntu.com/ops/latest/howto/write-unit-tests-for-a-charm/
self.harness = Harness(EndpointAggregatorCharm, meta=AGGREGATOR_META)

tests/unit/test_endpoint_aggregator.py::TestEndpointAggregatorWithRelabeling::test_adding_prometheus_then_target_with_bad_dns_keeps_input
tests/unit/test_endpoint_aggregator.py::TestEndpointAggregatorWithRelabeling::test_adding_prometheus_then_target_with_good_dns_adds_label
/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_endpoint_aggregator.py:634: PendingDeprecationWarning: Harness is deprecated. For the recommended approach, see: https://documentation.ubuntu.com/ops/latest/howto/write-unit-tests-for-a-charm/
self.harness = Harness(EndpointResolvingAggregatorCharm, meta=AGGREGATOR_META)

tests/unit/test_outgoing_relation_data.py::TestRelationMonitors::test_cos_agent_with_downstream_prometheus
tests/unit/test_outgoing_relation_data.py::TestRelationMonitors::test_only_cos_agent
tests/unit/test_outgoing_relation_data.py::TestRelationMonitors::test_only_prometheus
/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_outgoing_relation_data.py:82: PendingDeprecationWarning: Harness is deprecated. For the recommended approach, see: https://documentation.ubuntu.com/ops/latest/howto/write-unit-tests-for-a-charm/
self.harness = Harness(COSProxyCharm)

tests/unit/test_relation_monitors.py::TestRelationMonitors::test_monitors_changed
tests/unit/test_relation_monitors.py::TestRelationMonitors::test_prometheus
/home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit/test_relation_monitors.py:54: PendingDeprecationWarning: Harness is deprecated. For the recommended approach, see: https://documentation.ubuntu.com/ops/latest/howto/write-unit-tests-for-a-charm/
self.harness = Harness(COSProxyCharm)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== short test summary info ============================
FAILED tests/unit/test_charm.py::COSProxyCharmTest::test_alert_rules_are_forwarded_on_adding_prometheus_then_targets - AssertionError: Element counts were not equal:
FAILED tests/unit/test_charm.py::COSProxyCharmTest::test_alert_rules_are_forwarded_on_adding_targets_then_prometheus - AssertionError: Element counts were not equal:
FAILED tests/unit/test_charm.py::COSProxyCharmTest::test_alert_rules_removal_differentiates_between_applications - AssertionError: Element counts were not equal:
FAILED tests/unit/test_charm.py::COSProxyCharmTest::test_multiple_alert_rules_are_forwarded - AssertionError: Element counts were not equal:
FAILED tests/unit/test_charm.py::COSProxyCharmTest::test_removing_alert_rules_differentiates_between_units - AssertionError: Element counts were not equal:
================== 5 failed, 40 passed, 53 warnings in 2.35s ===================
unit: exit 1 (6.31 seconds) /home/runner/work/cos-proxy-operator/cos-proxy-operator> uv run --frozen --isolated --extra=dev coverage run --source=/home/runner/work/cos-proxy-operator/cos-proxy-operator/src -m pytest /home/runner/work/cos-proxy-operator/cos-proxy-operator/tests/unit pid=3058
unit: FAIL code 1 (7.50=setup[0.88]+cmd[0.30,6.31] seconds)
evaluation failed :( (7.53 seconds)
Error: Process completed with exit code 1.
```

### Additional context

_No response_