Reduce MCP tool-list trace volume

[jif-oai]

Why

Every MCP tool-list build emitted two normal-path TRACE events per configured server: one before waiting for tools and one after listing them. On active sessions this produced thousands of nearly identical SQLite rows while carrying little information beyond server readiness and tool counts.

What changed

• Remove the two normal-path per-server TRACE events.
• Keep the existing per-server trace span for timing and remote trace context.
• Emit per-server details only when a server's tools are unavailable.
• Emit one bounded summary per tool-list build with available server, unavailable server, and tool counts.

Successful builds retain the useful aggregate signal without repeating it for every server.

Related to #28224.