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.

coderd/aibridged/server.go

@@ -0,0 +1,9 @@
+package aibridged
+
+import "github.com/coder/coder/v2/coderd/aibridged/proto"
+
+type DRPCServer interface {
+	proto.DRPCRecorderServer
+	proto.DRPCMCPConfiguratorServer
+	proto.DRPCAuthorizerServer
+}