Fix UnicodeDecodeError in diskcache TAGS_CACHE.get()

[314clay]

Summary

Fixes #4729

TAGS_CACHE.get() in aider/repomap.py can crash with UnicodeDecodeError (or pickle.UnpicklingError / ValueError) when diskcache encounters corrupted cache entries. The existing tags_cache_error() recovery mechanism only caught SQLITE_ERRORS (sqlite3 and OS errors), missing these deserialization failures.

Changes:

• aider/repomap.py: Add import pickle and expand the SQLITE_ERRORS tuple to include UnicodeDecodeError, pickle.UnpicklingError, and ValueError — all existing except clauses already catch SQLITE_ERRORS and call tags_cache_error() for cache recreation, so this is a one-line fix
• tests/basic/test_repomap.py: Add TestTagsCacheErrorRecovery with 4 tests:
  • test_unicode_decode_error_in_cache_get — poisons TAGS_CACHE.get() with UnicodeDecodeError, verifies recovery
  • test_unpickling_error_in_cache_get — same with pickle.UnpicklingError
  • test_value_error_in_cache_get — same with ValueError
  • test_cache_errors_in_sqlite_errors_tuple — verifies all three types are in SQLITE_ERRORS

Test plan

• pytest tests/basic/test_repomap.py — all 49 tests pass (45 existing + 4 new)

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