fix: Avoid infinite loop when cross-reference keyspace is exhausted

[jonashaag]

Full disclosure: Written by GPT 5.6 Sol.

This fixes a problem with our docs build.

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Cross-reference stash keys start with _ and preserve the rendered identifier length. One-character and two-character identifiers therefore share a keyspace of only 62 keys. Once all keys are present, _gen_stash_key retries random choices forever.

This became reproducible for attribute values after 2.0.5 started rendering their source-level names: a large value containing more than 62 references through the same one-character module alias hangs the documentation build.

Detect an exhausted key length and expand subsequent keys by one character. Non-exhausted keyspaces retain the existing same-length behavior.

Validation:

• Focused rendering tests: 42 passed
• Full test suite: passed
• Downstream strict MkDocs build with 75 one-character alias references: passed

[jonashaag]

CI follow-up: the unrelated matrix failures are fixed in separate commits. Snapshot fixtures are checked out with LF on Windows; the docs inventory is only consumed by highest-resolution jobs; the global PyO3 ABI3 override was removed so current Pydantic builds on Python 3.15; and lowest-direct is limited to Python 3.10–3.13 because the minimum Pydantic stack cannot build on 3.14/3.15. The latest run is 31/31 green: https://github.com/mkdocstrings/python/actions/runs/29655904249