config.patch unconditionally writes the config file and sends SIGUSR1
even when diffConfigPaths detects zero changed paths. This causes a
full gateway restart (~10s downtime, all SSE/WebSocket connections
dropped) on every control-plane config.patch call, even when the
config is identical — e.g. a model hot-apply that doesn't change any
gateway.* paths.
Fix: when changedPaths is empty, return early with `noop: true`
without writing the file or scheduling SIGUSR1. The validated config
is still returned so the caller knows the current state.
This lets control-plane clients safely call config.patch for
idempotent updates without triggering unnecessary restarts.
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