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/utils_test.go

@@ -0,0 +1,27 @@
+package aibridged_test
+
+import (
+	"net/http"
+	"sync/atomic"
+
+	"go.opentelemetry.io/otel"
+)
+
+var testTracer = otel.Tracer("aibridged_test")
+
+var _ http.Handler = &mockAIUpstreamServer{}
+
+type mockAIUpstreamServer struct {
+	hitCounter atomic.Int32
+}
+
+func (m *mockAIUpstreamServer) ServeHTTP(rw http.ResponseWriter, _ *http.Request) {
+	m.hitCounter.Add(1)
+
+	rw.WriteHeader(http.StatusTeapot)
+	_, _ = rw.Write([]byte(`i am a teapot`))
+}
+
+func (m *mockAIUpstreamServer) Hits() int32 {
+	return m.hitCounter.Load()
+}