feat: implement document ai functions

[Ludwig-LMY]

Summary

Implement Document AI functions for the CCF/OceanBase training task.

• Add LOAD_FILE(location_name, file_name) to read local files from file:// LOCATION objects and return BLOB data.
• Add AI_SPLIT_DOCUMENT(content, parameters) as a table function.
• Support text/markdown document splitting by word or sentence with max and overlap parameters.
• Expose AI_SPLIT_DOCUMENT output columns:
  • chunk_id
  • chunk_offset
  • chunk_length
  • chunk_text

Tests

• Not run locally. This Windows environment does not have a full seekdb build/mysqltest runtime.
• Target mysqltest cases:
  • tools/deploy/mysql_test/test_suite/ai_funcs/t/load_file.test
  • tools/deploy/mysql_test/test_suite/ai_funcs/t/ai_split_document.test

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