Show return type annotations in --fixtures and --fixtures-per-test

[IslamElsayed]

Closes #13676.

Fixtures with a return annotation now show it in --fixtures and --fixtures-per-test:

answer -> int -- test_x.py:4
    Answer to the Ultimate Question of Life, the Universe, and Everything.

This picks up where #13680 left off (thanks @kianelbo), following what was agreed there:

• On Python 3.14+ the annotation is read as a string with annotationlib.get_annotations(func, format=Format.STRING), as @The-Compiler suggested. Nothing is evaluated, which covers @RonnyPfannschmidt's point about unresolvable annotations: a name imported only under TYPE_CHECKING is shown, not raised.
• Before 3.14 it uses inspect.formatannotation, with no regex post-processing, per @nicoddemus. It passes base_module=func.__module__, so a class from the fixture's own module shows as Answer rather than test_x.Answer. String annotations (from __future__ import annotations) are shown as written.
• -> None is shown as None, and fixtures without a return annotation are unchanged.

Built-in fixtures now show their annotations too (tmp_path -> Generator[Path]), so the existing TestShowFixtures expectations are updated. The new tests use pytester modules without the __future__ import, so the object path is covered on older Pythons, plus one test for an unresolvable TYPE_CHECKING name with the import and one, run on 3.14+ only, without it. I ran the affected tests on 3.10, 3.13 and 3.14.

I left the display format as in the issue (name -> type -- path) and only touched --fixtures and --fixtures-per-test.

AI assistance (Claude) was used while preparing this; I've reviewed the change, understand it, and will handle the review myself.