chore: nuke tests/ and the entire test toolchain

The test suite was carrying migration scars and a long tail of
low-density assertions over SDK-derived behavior. Drop it wholesale.

- Delete ``tests/`` (42 files, ~4900 LoC).
- Drop ``pytest`` / ``pytest-asyncio`` / ``pytest-cov`` /
  ``pytest-mock`` from the dev dependency group; ``uv sync``
  uninstalls the matching wheels.
- Strip the pytest + coverage config blocks, the
  ``flake8-pytest-style`` ruff selector, the ``tests/**`` per-file
  ignores, the ``[tool.mypy.overrides] tests.*`` block, and the
  ``"tests"`` entry from bandit's ``exclude_dirs``.
- Drop the ``test`` / ``test-cov`` Makefile targets; ``dev`` no
  longer depends on tests.
- Strip the ``# Testing`` block from ``.gitignore`` (``.coverage``,
  ``.pytest_cache/``, ``htmlcov/``, ``coverage.xml``, ``nosetests.xml``,
  ``.tox/``, ``.hypothesis/``).

ruff (27) and mypy (82) baselines unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -39,18 +39,6 @@ pip-delete-this-directory.txt
 .pydevproject
 .settings/
 
-# Testing
-.tox/
-.coverage
-.coverage.*
-.cache
-nosetests.xml
-coverage.xml
-*.cover
-.hypothesis/
-.pytest_cache/
-htmlcov/
-
 # FastAPI
 .env.local
 .env.development.local