feat: add /api/v2/ai-gateway API route aliases (#26475)

## Description

Registers `/api/v2/ai-gateway/*` as the new API path for AI Gateway, replacing `/api/v2/aibridge/*`. Both prefixes share the same route builder (`aiBridgeRoutes`) backed by a single in-memory handler, so existing `/aibridge` endpoints continue to work. New endpoints must be registered on the enterprise API handler under `/api/v2/ai-gateway` only.

Swagger annotations now point to `/api/v2/ai-gateway` paths with a backward-compatibility note referencing `/aibridge`. The legacy `/aibridge` routes are skipped in the swagger documentation test.

## Changes

- Store one raw handler (`aiGatewayHandler`) instead of two prefix-stripped handlers
- Register `/ai-gateway` and `/ai-gateway/proxy` route aliases alongside legacy `/aibridge` routes
- Move `/aibridge/keys` to `/ai-gateway/keys`
- Update in-process transport to use `/api/v2/ai-gateway` prefix
- Update SDK client URLs and proxy forwarding URL
- Swap `@Router` and `@Tags` annotations from `aibridge`/`AI Bridge` to `ai-gateway`/`AI Gateway`
- Rename user-facing error messages from "AI Bridge" to "AI Gateway"
- Define consts for route prefixes (`AIGatewayRootPath`, `AIBridgeRootPath`)
- Update tests and comments to use new paths

Note: the following will be addressed in follow-up PRs:
- Frontend API URLs
- Frontend routes and redirects
- Dogfood main.tf updates
- Hand-written documentation URL updates
- aibridge internal comments and nits
- Scale tests path updates

Refs https://linear.app/coder/issue/AIGOV-230

> Generated with the assistance of Coder Agents (@ssncferreira)

enterprise/coderd/aibridge_reload_test.go

@@ -54,7 +54,7 @@ func newMockUpstream(t *testing.T, name string) *mockUpstream {
 
 // startTestAIBridgeDaemon wires an in-process aibridged daemon onto
 // the supplied API and subscribes it to ai_providers change events.
-// This mirrors what cli/server.go does in production so /api/v2/aibridge
+// This mirrors what cli/server.go does in production so /api/v2/ai-gateway
 // requests dispatch through the real pool and reloader.
 func startTestAIBridgeDaemon(t *testing.T, api *coderd.API) *aibridged.Metrics {
 	t.Helper()
@@ -109,7 +109,7 @@ func (r *testPoolReloader) Reload(ctx context.Context) error {
 // TestAIBridgeProviderHotReload exercises the end-to-end CRUD ->
 // reload -> routing path: every provider mutation made through codersdk
 // must, within a short window, change the routing observed at
-// /api/v2/aibridge/{name}/v1/models. The OpenAI passthrough route
+// /api/v2/ai-gateway/{name}/v1/models. The OpenAI passthrough route
 // /v1/models reverse-proxies to BaseURL, so the upstream that responds
 // identifies which provider the daemon's mux dispatched to.
 func TestAIBridgeProviderHotReload(t *testing.T) {
@@ -162,11 +162,11 @@ func TestAIBridgeProviderHotReload(t *testing.T) {
 
 	ctx := testutil.Context(t, testutil.WaitLong)
 
-	// sendRequest issues GET /api/v2/aibridge/{name}/v1/models and
+	// sendRequest issues GET /api/v2/ai-gateway/{name}/v1/models and
 	// returns the status and the upstream marker decoded from the
 	// JSON body (empty if the body was not the marker JSON).
 	sendRequest := func(providerName string) (int, string) {
-		url := client.URL.String() + "/api/v2/aibridge/" + providerName + "/v1/models"
+		url := client.URL.String() + "/api/v2/ai-gateway/" + providerName + "/v1/models"
 		req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 		require.NoError(t, err)
 		req.Header.Set("Authorization", "Bearer "+client.SessionToken())