Type the optional readlines size hint - #3541

Type checkers currently reject await file.readlines(10) even though the wrapped synchronous file and Trio's runtime accept the optional size hint. Add the positional-only hint: int = -1 parameter to AsyncIOWrapper.readlines.

The type regressions cover text and binary results and positive/default hints. A runtime regression verifies that the hint is forwarded and subsequent reads return the remaining lines. A news fragment is included.

Validation:

• The new calls produce five mypy errors before the annotation change and pass afterward; targeted pyright reports zero errors.
• All 18 file I/O tests pass, and the installed-wheel suite passes with the CI plugin ordering (826 passed, 71 skipped, 2 xfailed).
• Configured mypy passes for all 144 source files, and all applicable pre-commit checks pass.
• The whole-repository mypy invocation has one unchanged error in docs/source/conf.py with this environment's Sphinx version; the same error is reproduced on the original base.