feat(mcp): allow explicit reranker provider selection

[abouchard11]

Summary

Add an explicit MCP reranker.provider configuration while preserving the automatic selection behavior merged in #1637.

• auto keeps the current LLM → embedder → local BGE fallback chain.
• openai, azure_openai, and gemini reuse credentials already configured under the matching LLM/embedder provider entry.
• bge explicitly selects the existing local reranker.
• Document both YAML and direct environment overrides.

This is the explicit configuration follow-up discussed around #1637 and the earlier provider-matched proposal #1669.

Closes #1697.

Testing

• Test-first coverage for configuration parsing, explicit Gemini override behavior, and Graphiti service wiring.
• 68 passed across MCP factory, configuration, core-parity, transport, and stdio unit tests.
• ruff check and ruff format --check pass for all changed Python files.
• Focused Pyright passes with 0 errors, 0 warnings.

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