fix: catch OSError from Path.is_dir() with invalid filenames

[12310312long]

Summary

On macOS/Linux, Path.is_dir() can raise OSError with ENAMETOOLONG (Errno 63) when the path is excessively long or malformed (e.g., when a long --system-prompt value ends up as a positional argument). Three locations in main.py call Path(fname).is_dir() without protection, causing unhandled crashes.

This fix wraps each call site in try/except OSError and provides a user-friendly warning instead of a traceback.

Changes

• aider/main.py:
  • read_only_fnames loop: wrap .expanduser().resolve() and .is_dir() in try/except, skip invalid paths with a warning
  • Multi-file validation: wrap Path(fname).is_dir() in try/except, treat OSError as an invalid path
  • Single-directory detection: wrap Path(all_files[0]).is_dir() and .resolve() in try/except, error gracefully on failure

Related Issues

Closes #5130 — Uncaught OSError in pathlib.py line 1222
Closes #5082 — Uncaught OSError in pathlib.py line 841

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