refactor: move aibridged out of enterprise to AGPL (#25570)

In order to allow Coder Agents to use AI Gateway in OSS, we need to rehome the `aibridged`\-related code into the AGPL path.

The HTTP API is only registered under enterprise so will still require the AI Governance Add-on to be present in order to use it, whereas Coder Agents uses an in-memory pipe to the same handlers.

enterprise/coderd/coderd.go

@@ -797,7 +797,6 @@ type API struct {
 	licenseMetricsCollector *license.MetricsCollector
 	tailnetService          *tailnet.ClientService
 
-	aibridgedHandler      http.Handler
 	aibridgeproxydHandler http.Handler
 	aiSeatTracker         *aiseats.SeatTracker
 }