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)

coderd/aibridged.go

@@ -19,13 +19,12 @@ import (
 	"github.com/coder/coder/v2/codersdk/drpcsdk"
 )
 
-// GetAIBridgedHandler returns the in-memory aibridge HTTP handler set by
-// [API.RegisterInMemoryAIBridgedHTTPHandler], or nil if the daemon has not
-// been wired in. Used by the enterprise /api/v2/aibridge route (license-gated)
-// to forward requests into the same in-memory handler that chatd dispatches
-// to in-process.
-func (api *API) GetAIBridgedHandler() http.Handler {
-	return api.aibridgedHandler
+// AIGatewayHandler returns the in-memory AI Gateway HTTP handler
+// set by [API.RegisterInMemoryAIBridgedHTTPHandler], or nil if the daemon
+// has not been wired in. Callers must apply their own [http.StripPrefix]
+// for the route prefix they are mounting under.
+func (api *API) AIGatewayHandler() http.Handler {
+	return api.aiGatewayHandler
 }
 
 // RegisterInMemoryAIBridgedHTTPHandler mounts [aibridged.Server]'s HTTP router onto
@@ -42,9 +41,9 @@ func (api *API) RegisterInMemoryAIBridgedHTTPHandler(srv http.Handler) {
 		panic("aibridged cannot be nil")
 	}
 
-	api.aibridgedHandler = http.StripPrefix("/api/v2/aibridge", srv)
+	api.aiGatewayHandler = srv
 
-	factory := aibridged.NewTransportFactory(api.aibridgedHandler)
+	factory := aibridged.NewTransportFactory(http.StripPrefix(agplaibridge.AIGatewayRootPath, srv))
 	var asInterface agplaibridge.TransportFactory = factory
 	api.AIBridgeTransportFactory.Store(&asInterface)
 }