[FEAT]: Deprecate the rampart_sinks fixture in favor of the pytest_rampart_sinks hook

[nina-msft]

Summary

Follow-up to #73. Now that the pytest_rampart_sinks hook is the supported
sink-registration API, this PR begins deprecating the legacy rampart_sinks
fixture.

Resolving the fixture now emits a DeprecationWarning (removal target: 0.3.0)
in both fixture-consumption paths:

• Single-process — _rampart_sink_bootstrap, after the fixture resolves.
• xdist controller — _resolve_sink_candidate, when it unwraps a genuine
  @pytest.fixture-form rampart_sinks (not module-level lists or plain
  callables).

The hook stays authoritative and the fixture remains a single-process fallback,
so nothing breaks for existing users — they just get a migration nudge.

Changes

• New rampart/common/deprecation.py — emit_deprecation_warning(...),
  mirroring PyRIT's helper (qualified-name resolution + DeprecationWarning at
  stacklevel=3).
• Wire it into the plugin.py and _xdist.py fixture-resolution paths.
• Docs: deprecation admonitions in usage/pytest-integration.md and
  usage/xdist.md.
• Tests: tests/unit/common/test_deprecation.py (unit) and a
  TestSinkFixtureDeprecation class in the slow aggregation suite that drives
  real subprocess pytest runs via pytester — single-process warn,
  xdist-controller warn, and list-form no-warn — replacing the previous
  MagicMock-based warn/no-warn tests.

Breaking changes

None. The fixture continues to work as a single-process fallback; it only emits
a DeprecationWarning now.

Validation

• uv run ruff check . → all checks pass; ruff format --check clean.
• uv run ty check → all checks passed.
• uv run pytest tests/unit/common/test_deprecation.py tests/unit/pytest_plugin/test_xdist.py tests/unit/pytest_plugin/test_plugin.py → 145 passed.
• uv run pytest tests/unit/pytest_plugin/test_xdist_aggregation.py -m slow → 16 passed (includes the pytester-based TestSinkFixtureDeprecation class).

Notes

• removed_in="0.3.0" is the removal milestone for the fixture — deprecated in
  the 0.1 line, kept through 0.2, removed in 0.3 (two minor versions, per
  docs/contributing/release-process.md).
• Implements the first of the post-merge deprecation follow-ups raised by
  @bashirpartovi.

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes
• Documentation updated

[spencrr]

Looks good! First quick pass:

• print_deprecation_message -> log deprecation warning maybe?
• teats have a lot of mocks -> let’s use pytester and capture warnings that way?

[nina-msft]

@spencrr I just updated the test name to be emit_deprecation_warning before seeing your comment so that is already addressed :D I'll do another pass through the tests shortly!