Summary

Fixes #3594 — onToolsChanged was written and read from MCP
change-notification goroutines with no synchronization, while its sibling
onBackgroundEvent is already mutex-guarded (backgroundEventMu) with a
comment explaining exactly this hazard.

What changed

• onToolsChanged is now guarded by a new toolsChangedMu sync.RWMutex,
  faithfully mirroring the onBackgroundEvent/backgroundEventMu precedent:
  the setter writes under the write lock; emitToolsChanged copies the handler
  under the read lock and then releases it before reading tools,
  constructing the event, and invoking the handler (never holding the mutex
  across the callback).

Testing

• pkg/runtime/ontoolschanged_race_test.go — concurrent register + emit.
  Verified to fail (data race at the setter write + the two reads) against the
  pre-fix unguarded code and pass now.
• task dev + go test -race -count=2 ./pkg/runtime/...: green (validated in
  isolation).