fix(mcp): make open(path) round-trip and return not_found details

[Razpines]

Summary

• make DocStore.open_doc(path=...) resolve docs by exact/normalized origin_path
• add canonical URL lookup support in open_doc(path=...)
• keep doc-id lookup behavior and add path-derived fallback doc-id normalization
• change MCP open() missing-doc response to structured
  ot_found payload with attempted inputs
• add tests covering origin_path round-trip and canonical URL lookup

Issues

• Closes bug(mcp): make open(path=origin_path) round-trip from search results #24

Testing

• python -m py_compile src/unity_docs_mcp/mcp_server.py src/unity_docs_mcp/tools/ops.py tests/test_mcp_meta.py tests/test_ops_open_path_lookup.py
• $env:PYTHONPATH='C:\projects\UnityRAG\src'; python -m pytest tests/test_ops_open_path_lookup.py -q