[codex] support TiDB Vector full-text and hybrid search

[bkidy]

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Summary

Fixes #37145

This PR adds optional full-text and hybrid search support for TiDB Vector.

• Adds TIDB_VECTOR_ENABLE_FULLTEXT_SEARCH with a default of false so existing TiDB Vector deployments remain semantic-only by default.
• Creates a TiDB FULLTEXT INDEX ... WITH PARSER MULTILINGUAL on the text column when the feature is enabled.
• Implements search_by_full_text with FTS_MATCH_WORD, including document-id filtering and score propagation.
• Exposes full-text and hybrid retrieval methods for TiDB Vector only when the feature flag is enabled.
• Adds unit coverage for the provider and retrieval-method contract.

Note: I attempted to assign issue #37145 to myself, but GitHub returned bkidy not found for the upstream issue assignment. The issue is still linked here.

Screenshots

Before	After
N/A	N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

Validation

• PYTHONPATH=api/providers/vdb/vdb-tidb-vector/src api/.venv/bin/python -m pytest -o addopts='' api/providers/vdb/vdb-tidb-vector/tests/unit_tests/test_tidb_vector.py api/tests/unit_tests/controllers/console/datasets/test_datasets.py::TestDatasetRetrievalSettingApi::test_tidb_vector_returns_semantic_only_when_fulltext_disabled api/tests/unit_tests/controllers/console/datasets/test_datasets.py::TestDatasetRetrievalSettingApi::test_tidb_vector_returns_full_methods_when_fulltext_enabled -q
• git diff --check
• ruff format --check and ruff check on the touched Python files

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