MCP upgrade 2/6: Revision-aware core service

[Lauritz-Timm]

Part of the six-phase full MCP service upgrade.

Why

The service needs one revision-aware protocol core that keeps legacy clients working while modern clients receive validated capabilities and tool metadata.

What

• Adds the typed MCP tool catalog and revision negotiation.
• Centralizes modern request validation and protocol responses.
• Preserves legacy dispatch compatibility.
• Provides the direct foundation for the typed-output, project-context, and HTTP-proxy phases.

Dependency

Standalone from develop. Phases 3, 4, and 6 depend directly on this phase; each remains independently mergeable after Core.

Verification

• Formatting and strict all-target/all-feature Clippy passed.
• Integrated workspace tests passed: CLI 439/439 (2 opt-in model-download tests ignored), Core 56/56, MCP 135/135, Store 222/222 (2 ignored), OpenSearch 5/5, PostgreSQL 4/4.
• Final isolated candidate evaluation passed 294/294 scenarios in each of two portable roots with normalized equality.