fix: handle PermissionError in version check cache write

[scinttt]

Problem

check_version() crashes with an uncaught PermissionError when the version cache file (~/.aider/caches/versioncheck) cannot be written. This prevents aider from starting entirely in restricted environments (read-only filesystems, Docker containers with --read-only, SELinux/AppArmor policies, NFS with root_squash).

Traceback from the issue:

File "versioncheck.py", line 95, in check_version
    VERSION_CHECK_FNAME.touch()
PermissionError: [Errno 13] Permission denied: '/root/.aider/caches/versioncheck'

Root Cause

The finally block in check_version() calls mkdir() and touch() without any exception handling. Since finally runs unconditionally, a PermissionError (or any OSError) propagates as an uncaught exception and crashes aider.

Fix

Wrap the finally block's filesystem operations in try/except OSError. Using OSError (parent of PermissionError) also handles other filesystem issues like read-only mounts and disk-full scenarios. A silent pass is appropriate here since the version cache is non-critical — the only consequence of failure is that the next launch will re-check PyPI, which is harmless.

Tests

Added tests/basic/test_versioncheck.py with 4 test cases:

• PermissionError on touch() does not crash
• PermissionError on mkdir() does not crash
• Generic OSError (e.g. read-only filesystem) does not crash
• Normal version check behavior is preserved

Fixes #4958

[CLAassistant]

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