Remove the dead `cloudant_fake` module, or add its missing tests

## Description

| Module | Expected test file |
| --- | --- |
| `providers/cloudant/src/airflow/providers/cloudant/cloudant_fake.py` | `providers/cloudant/tests/unit/cloudant/test_cloudant_fake.py` |

It is currently suppressed in the `OVERLOOKED_TESTS` allowlist in [`airflow-core/tests/unit/always/test_project_structure.py`](https://github.com/apache/airflow/blob/main/airflow-core/tests/unit/always/test_project_structure.py).

This is a scoped subset of the meta issue #35442, limited to the `cloudant` provider.

### What should these tests cover?

Probably nothing, and that is the finding worth recording here. `cloudant_fake.py` defines two no-op stand-ins, `CloudantV1` and `CouchDbSessionAuthenticator`, whose own docstrings describe them as a "Phony class to pass mypy when real class is not imported". Every method body is empty, so there is no behaviour a test could meaningfully assert.

More to the point, the module is dead. It existed to serve one conditional import in the hook:

```python
if sys.version_info < (3, 10):
from airflow.providers.cloudant.cloudant_fake import CloudantV1, CouchDbSessionAuthenticator
else:
from ibmcloudant import CloudantV1, CouchDbSessionAuthenticator
```

That fallback was needed while `ibmcloudant` was excluded on Python 3.9, as recorded in the `excluded-python-versions` block that used to sit in `providers/cloudant/provider.yaml`. #52072 dropped Python 3.9 support, removed the version guard, and left the hook importing `CloudantV1` and `CouchDbSessionAuthenticator` directly from `ibmcloudant`. The stub module was not removed with it.

A search across the repository now finds no import of `airflow.providers.cloudant.cloudant_fake` in any source, test, or configuration file. Its only remaining reference anywhere is its own entry in `OVERLOOKED_TESTS`.

So the sensible resolution is removal rather than new tests:

1. Delete `providers/cloudant/src/airflow/providers/cloudant/cloudant_fake.py` and drop its `OVERLOOKED_TESTS` entry.
2. If it turns out the module is deliberately retained for a mypy path that is not visible from the source, keep it and add a minimal `test_cloudant_fake.py` asserting both classes are constructible and that `set_service_url` accepts a URL, then drop the allowlist entry either way.

Option 1 looks correct given the history above, but it is worth a second opinion from anyone with context on the original optional-import arrangement before the file is deleted.

## Definition of Done

1. Confirm whether `cloudant_fake.py` is still needed by anything.
2. Either delete the module, or add `providers/cloudant/tests/unit/cloudant/test_cloudant_fake.py`.
3. Remove the `providers/cloudant/tests/unit/cloudant/test_cloudant_fake.py` entry from `OVERLOOKED_TESTS` in `airflow-core/tests/unit/always/test_project_structure.py`
4. These should both pass, the second one confirming that nothing depended on the stub for typing:

```bash
breeze testing providers-tests providers/cloudant/tests/unit/cloudant

breeze run mypy providers/cloudant/src/airflow/providers/cloudant
```

---
Drafted-by: Claude Code (Opus 5); reviewed and edited by @jroachgolf84 before posting.

Start by reviewing providers/cloudant/src/airflow/providers/cloudant/cloudant_fake.py and searching the repository for references to it, then inspect the OVERLOOKED_TESTS entry in airflow-core/tests/unit/always/test_project_structure.py. Confirm the module is unnecessary, remove it and its allowlist entry, and run the provider tests plus the specified mypy command to verify the cleanup.

Written by the indexing model from the issue text.