[ty] Avoid unnecessary Salsa queries during pytest fixture lookup

[lerebear]

Summary

In anticipation of "find references" for pytest fixtures, which introduces many more speculative attempts at resolving pytest fixtures, this PR helps preserve efficiency by short-circuiting a few Salsa queries:

• Return from fixture_declaration before calling function_known_decorators when a function has no decorators.
• Return from fixture_candidates_from_definition before calling exists_at_runtime when the definition kind cannot expose a fixture.
• Return from directly_parametrized before calling function_known_decorators when the test function has no decorators (while still checking parametrization from an enclosing class).

These are all behavior-preserving early exits.

Test Plan

This is a refactor that relies on existing test coverage.