ci: Fix test matrix compatibility

[jonashaag]

For reviewers

• I did not use AI
• I used AI and thoroughly reviewed every code/docs change

Description of the change

Fix unrelated CI matrix failures discovered while validating #334:

• Keep snapshot fixtures on LF line endings on Windows.
• Download the documentation inventory only in highest-resolution jobs, which are the only jobs that consume it.
• Remove the global PyO3 ABI3 override so current Pydantic releases build on Python 3.15.
• Exclude Python 3.14 and 3.15 from lowest-direct, because the minimum supported Pydantic stack does not build on those Python versions.

Relevant resources

• Split out of fix: Avoid infinite loop when cross-reference keyspace is exhausted #334 at the maintainer's request.
• Green 31-job validation run before the split: https://github.com/mkdocstrings/python/actions/runs/29655904249

[jonashaag]

I used AI and did not thoroughly review the changes.

[pawamoy]

I'll close then, thanks for being honest about it.