[data_interfaces][v1] SecretNotFound raised sometimes on setting content

This might be a juju problem.

## Steps to reproduce
The issue is flaky and can be seen [here](https://github.com/canonical/charmed-etcd-operator/actions/runs/19689344836/job/56402017761#step:13:15904), and [here](https://github.com/canonical/k8s-operator/actions/runs/19680258154/job/56383866896#step:25:1855)

## Expected behavior
We should find out why the secret is not being found even though it should be there. Our caching is hiding problem a little because we do not get the secret from juju in the `serialize_model` function but instead return it from the cache. We only find the problem when calling ops's `set_content` on the actual juju secret.

## Actual behavior

```
unit-charmed-etcd-0: 01:50:03 DEBUG unit.charmed-etcd/0.juju-log etcd-peers:3: charms.data_platform_libs.v1.data_interfaces:Secret with label etcd-client.7.fa2f173949a38a15.mtls.secret not found
unit-charmed-etcd-0: 01:50:03 ERROR unit.charmed-etcd/0.juju-log etcd-peers:3: root:Uncaught exception while in charm code:
Traceback (most recent call last):
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 3637, in _run
result = subprocess.run(args, **kwargs) # type: ignore
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/subprocess.py", line 571, in run
raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('/var/lib/juju/tools/unit-charmed-etcd-0/secret-info-get', '--label', 'etcd-client.7.user.secret', '--format=json')' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 4034, in _run_for_secret
return self._run(*args, return_output=return_output, use_json=use_json)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 3639, in _run
raise ModelError(e.stderr) from e
ops.model.ModelError: ERROR secret "etcd-client.7.user.secret" not found

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/lib/charms/data_platform_libs/v1/data_interfaces.py", line 891, in serialize_model
secret.set_content(full_content)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/lib/charms/data_platform_libs/v1/data_interfaces.py", line 606, in set_content
self.meta.set_content(content)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 1536, in set_content
self._id = self.get_info().id
^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 1513, in get_info
return self._backend.secret_info_get(id=self.id, label=self.label)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 4046, in secret_info_get
result = self._run_for_secret('secret-info-get', *args, return_output=True, use_json=True)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 4037, in _run_for_secret
raise SecretNotFoundError() from e
ops.model.SecretNotFoundError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/src/charm.py", line 415, in
ops.main(EtcdOperatorCharm) # type: ignore
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/__init__.py", line 356, in __call__
return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 504, in main
manager.run()
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 488, in run
self._emit()
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 423, in _emit
self._emit_charm_event(self.dispatcher.event_name)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 467, in _emit_charm_event
event_to_emit.emit(*args, **kwargs)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 351, in emit
framework._emit(event)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 924, in _emit
self._reemit(event_path)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 1030, in _reemit
custom_handler(event)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/src/events/etcd.py", line 359, in _on_peer_relation_changed
self._update_client_relations()
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/src/events/etcd.py", line 769, in _update_client_relations
self.charm.external_clients_manager.update_client_relations_data(
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/src/managers/external_clients.py", line 157, in update_client_relations_data
self.state.etcd_provides_event_handler.set_responses(relation.id, responses)
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/lib/charms/data_platform_libs/v1/data_interfaces.py", line 2540, in set_responses
self.interface.write_model(
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/lib/charms/data_platform_libs/v1/data_interfaces.py", line 1710, in write_model
write_model(
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/lib/charms/data_platform_libs/v1/data_interfaces.py", line 1790, in write_model
dumped = model.model_dump(
^^^^^^^^^^^^^^^^^
File "/var/lib/juju/agents/unit-charmed-etcd-0/charm/venv/lib/python3.12/site-packages/pydantic/main.py", line 463, in model_dump
return self.__pydantic_serializer__.to_python(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.PydanticSerializationError: Error calling function `serialize_model`: SecretNotFoundError:
```

Start by reading lib/charms/data_platform_libs/v1/data_interfaces.py around serialize_model, Secret.set_content, and write_model, then reproduce the failure using the two linked GitHub Actions runs. Trace why the cached secret is unavailable to Juju during set_content; done means identifying and preventing the intermittent SecretNotFoundError during model serialization.

Written by the indexing model from the issue text.