Refactor MCP gateway into focused modules

[lucarlig]

Summary

• rename mcp_gateway.rs to mcp_service.rs so the filename matches its primary responsibility
• keep mcp_service.rs as a thin McpService definition and RMCP ServerHandler adapter
• group method implementations under mcp_service/ by capability: initialization, tools, resources, prompts, and completion
• extract backend transport state into backend_transports.rs
• extract identifier namespacing and backend resolution into identifier_routing.rs
• extract list fan-out and result merging into list_aggregation.rs
• move unit tests beside the routing and aggregation logic they cover
• update the gateway book source-path reference

Why

The original module mixed RMCP server handling, session-scoped backend transport storage, identifier routing, list aggregation, and tests in one file. Separating those concerns makes the hot path easier to navigate while retaining one clear adapter for the RMCP trait.

Impact

This is a behavior-preserving internal refactor. MCP handler bodies now live behind thin ServerHandler delegations, with no intended protocol or runtime behavior changes.

Validation

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace (68 passed)
• mdbook build docs/book
• mdbook test docs/book
• git diff --check