[ty] Factor function-level work out of pytest fixture request lookup

[lerebear]

Summary

In anticipation of "find references" for pytest fixtures, which enumerates possible fixture requests across every parameter of a function, this PR factors the shared, function-level work in fixture_request_for_parameter into FixtureRequestContext.

This lets the follow-up construct one context per function and reuse it for each candidate parameter, rather than repeatedly calling:

• fixture_declaration, is_collected_test, and is_unittest_test_case to determine whether the function can request fixtures.
• infer_definition_types to retrieve the function type.
• mock_patch_count to determine how many parameters are supplied by unittest.mock.patch.

The existing fixture_request_for_parameter entry point now constructs a context and delegates to it, preserving behavior for existing callers. This is a behavior-preserving refactor.

Test Plan

This is a refactor that relies on existing test coverage (although I have backfilled a new test for some pre-existing behaviour).

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 97.79%. The percentage of expected errors that received a diagnostic held steady at 94.33%. The number of fully passing files held steady at 112/136.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)