feat(store): file and vector store support seekdb (embedded + remote)

[dengfuping]

Summary

Adds SeekdbVectorStore and SeekdbFileStore on pyseekdb, with embedded (optional path; omitted → library default) and remote OceanBase/seekdb via host / port only (no uri string). Shared connection building lives in reme/core/utils/pyseekdb_conn.py with shared defaults (2881, sys, root); helpers are re-exported from reme.core.utils.

Details

• Embedding upsert: coerce vectors to the configured dimension and reject empty lists before collection.upsert to avoid embedded SQL errors (1210).
• Tests: --seekdb for vector and file store suites; embedded uses path under temp db_path; optional remote via SEEKDB_HOST and optional SEEKDB_PORT (no SEEKDB_URI).
• Dependency: pyseekdb per pyproject.toml.

How to verify

python tests/test_vector_store.py --seekdb
python tests/test_file_store.py --seekdb

Relate issues

Closes #206

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