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/aigatewaykeys.go

@@ -28,7 +28,7 @@ const nameFormatDetail = "Must be 64 characters or fewer, lowercase letters, num
 // @Tags Enterprise
 // @Param request body codersdk.CreateAIGatewayKeyRequest true "Create AI Gateway key request"
 // @Success 201 {object} codersdk.CreateAIGatewayKeyResponse
-// @Router /api/v2/aibridge/keys [post]
+// @Router /api/v2/ai-gateway/keys [post]
 func (api *API) postAIGatewayKey(rw http.ResponseWriter, r *http.Request) {
 	var (
 		ctx               = r.Context()
@@ -116,7 +116,7 @@ func writeKeyInsertError(ctx context.Context, rw http.ResponseWriter, err error)
 // @Produce json
 // @Tags Enterprise
 // @Success 200 {array} codersdk.AIGatewayKey
-// @Router /api/v2/aibridge/keys [get]
+// @Router /api/v2/ai-gateway/keys [get]
 func (api *API) aiGatewayKeys(rw http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 
@@ -146,7 +146,7 @@ func (api *API) aiGatewayKeys(rw http.ResponseWriter, r *http.Request) {
 // @Tags Enterprise
 // @Param key path string true "Key ID" format(uuid)
 // @Success 204
-// @Router /api/v2/aibridge/keys/{key} [delete]
+// @Router /api/v2/ai-gateway/keys/{key} [delete]
 func (api *API) deleteAIGatewayKey(rw http.ResponseWriter, r *http.Request) {
 	var (
 		ctx               = r.Context()