test: expand test coverage

[Alexander-Auer]

test: expand unit test coverage to every logical case

Important

**Depends on #28 . Merge after #28 ** (PR 3 of 4; the full order is listed in #27).
Until #REGISTRY is merged, this diff also shows the earlier PRs. This PR's own change is one commit:
9dac6f5

This PR adds tests only, apart from enabling coverage measurement. No library code changes.

What is covered

Configuration: a field × kind matrix (test_config.py). Two tables define the five configuration shapes (arc, htcondor-ce, cloud, batch+local, batch+ssh) and which fields apply to each. From them, the tests generate every combination and check that:

• each field is refused, by name, wherever it doesn't apply;
• each optional field is accepted wherever it applies, and its value actually reaches the backend's settings;
• each required field is named when it is missing.

Two guard tests fail if a field or kind is added to BackendConfig without being added to the tables. There are also tests for:

• the defaults of each shape;
• every auth combination, valid and invalid;
• every number field refusing 0 and -1;
• choices being case-sensitive;
• error messages reporting every problem at once, without pydantic noise.

The fake backend (test_fake_backend.py):

• every operation × closed and unavailable;
• every per-job operation × unknown id, other resource, other backend, injected failure, and an empty batch;
• kill, output and diagnostics × every job status;
• what submit actually stores: local inputs read once at submit, URLs left to the resource, and per-copy secrets and environment;
• every staging and container accept/refuse case, purge followed by every other operation, and an escape attempt writing nothing at all.

Models and smoke tests:

• JobHandle.key escaping;
• defaults;
• that the JobStatus values stay stable, since stored statuses are compared by value;
• an unparametrised OpOutcome;
• the version fallback when the package is not installed.

Tooling

• pytest-cov in the testing extras.
• [tool.coverage]: branch coverage, subprocesses measured too, and protocol ... bodies and TYPE_CHECKING blocks excluded.

pixi run -e py313 pytest --cov

With this PR, line and branch coverage of src/intercede is 100%. A coverage gate (fail_under) is not added; whether to enforce one is a decision for the team.

477 tests pass on Python 3.11 and 3.13, and pre-commit passes.