fix(runtime): guard onToolsChanged like onBackgroundEvent

[aheritier]

Summary

pkg/runtime/runtime.go:307 onToolsChanged is written at :1431 and read at :1449/:1459 from MCP change-notification goroutines with no lock. The sibling onBackgroundEvent IS mutex-guarded (backgroundEventMu) with a comment explaining exactly this hazard.

Fix

Guard onToolsChanged identically (or make both atomic.Pointer).

Acceptance

• -race clean; consistent with the onBackgroundEvent precedent.