[MCP Migration] Deprecate and remove fastmcp after SDK engine rollout

[jolestar]

Parent: #485

Goal

Once the official MCP SDK engine is stable and defaulted, remove fastmcp from the production path to prevent protocol drift and reduce maintenance burden.

Scope

• Deprecate fastmcp engine (announce timeline, keep rollback window if needed).
• Remove fastmcp dependency where no longer required.
• Remove code paths and test configuration that special-case fastmcp (e.g., jest transform ignore patterns).
• Update docs/examples to use the official SDK engine by default.

Acceptance Criteria

• No runtime dependency on fastmcp for production services.
• Build/test passes.
• Documentation reflects the new default and rollback story.