Fix: Handle MySQL emoji encoding error (1366) in DAG code insertion

[MonsterChenzhuo]

This PR improves the handling of database errors when persisting DAG source code, specifically addressing the issue where emoji or other unsupported characters in DAG files cause MySQLdb.OperationalError (1366) on databases with utf8 charset.

Previously, the INSERT INTO dag_code operation was deferred and triggered during the subsequent FAB permission sync ( session.commit() ). This caused the error to be caught by FAB's internal error handler (logged as "Creation of Permission View Error") instead of being handled as a DAG serialization error. As a result, the parsing loop would fail without reporting a visible ImportError to the user.

Changes:

• Added explicit session.flush() in _serialize_dag_capturing_errors to force DB writes before entering permission sync.
• This ensures that OperationalError is raised within the scope of our try-except block, allowing us to catch the 1366 error and record it as a proper ImportError .
• Fixed a missing session argument in DagCode.update_source_code call.
• Updated tests to verify that errors raised during flush are correctly captured.

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