Fix AttributeError for removed DocumentStorage.connection in 4.5.0

[Copilot]

After upgrading to 4.5.0, users see 'DocumentStorage' object has no attribute 'connection' errors in logs when using knowledge base features. This occurs because the 4.5.0 refactor migrated DocumentStorage from synchronous (self.connection) to asynchronous operations (self.engine), but cached bytecode or outdated plugins still reference the old attribute.

Changes

• Add defensive @property connection to DocumentStorage that raises AttributeError with migration guidance:

  • Shell commands to clear Python cache files
  • Instructions to replace self.connection with self.engine and use async with self.get_session()
  • Reference to source code examples

• Update v4.5.1 changelog with fix reference

Example

Old code attempting to access the removed attribute now gets actionable feedback:

storage = DocumentStorage(db_path)
_ = storage.connection  # Raises:
# AttributeError: DocumentStorage.connection attribute was removed in AstrBot 4.5.0.
# The database now uses async operations with self.engine and async context managers.
# 
# If you're seeing this error, please:
# 1. Clear Python cache files: find data/plugins -name '*.pyc' -delete && ...
# 2. Update any custom plugins that use DocumentStorage:
#    - Replace 'self.connection' with 'self.engine'
#    - Use 'async with self.get_session() as session' for database operations
# 3. Restart AstrBot

The fix is defensive only—no functional changes to DocumentStorage operations.

Original prompt

#3191
修复这个issue

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