Bundle builder cannot fulfill discourse-k8s:oauth endpoint (UncompletableBundleError)

**Note: This issue was generated with AI assistance (GitHub Copilot) based on automated log analysis and triage.**
Filed by @canonical/solutions-qa

### Observed symptom

`test_build_bundle` fails repeatedly with:

```
bundle_builder_x.bundle_builder.UncompletableBundleError: Could not build a complete valid bundle: Cannot fulfill charm endpoints: discourse-k8s:oauth
```

### Context

- Target charm under test: `grafana-agent-k8s` (track `2`, version `221`)
- Test suite/case: `charm_integration_testing.test_suite.test_build_bundle` / `test_build_bundle`
- Environment: `juju:4/stable ubuntu:24.04 cmr/kubernetes/kubernetes`
- `discourse-k8s` is pulled in as a neighbor charm during bundle construction, and its `oauth` requires-endpoint cannot be fulfilled by the solver.
- This is a recurring failure, observed 6 times between 2026-07-13T14:14 and 2026-07-14T01:56 across separate test runs, always with the identical error message.

### Sample failures (Test Observer)

- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656622&testResultId=12418097
- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656620&testResultId=12416648
- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656562&testResultId=12416068
- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656566&testResultId=12416050
- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656564&testResultId=12411572
- https://test-observer.canonical.com/#/charms/409553?testExecutionId=656560&testResultId=12411554

### Steps to reproduce

Run `test_build_bundle` for `grafana-agent-k8s` such that `discourse-k8s` is included as a neighbor charm requiring the `oauth` interface, in the `juju:4/stable ubuntu:24.04 cmr/kubernetes/kubernetes` environment.

### Additional execution metadata captured

```
charm_qa:failure:build_bundle:unfulfilled_endpoint: discourse-k8s:oauth
charm_qa:failure:build_bundle:unfulfilled_interface: oauth
```

Start by running charm_integration_testing.test_suite.test_build_bundle::test_build_bundle for grafana-agent-k8s in the stated Juju, Ubuntu, and Kubernetes environment. Trace how discourse-k8s is included as a neighbor and why its oauth requires-endpoint remains unfulfilled. Done means the recurring UncompletableBundleError is resolved and the test builds a complete valid bundle.

Written by the indexing model from the issue text.