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/coderdtest/swaggerparser.go

@@ -13,6 +13,8 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/aibridge"
 )
 
 type SwaggerComment struct {
@@ -168,6 +170,14 @@ func isExperimentalEndpoint(route string) bool {
 	return strings.HasPrefix(route, "/api/v2/workspaceagents/me/experimental/")
 }
 
+// isLegacyAIBridgeAlias returns true for /api/v2/aibridge routes that are
+// backward-compatibility aliases of /api/v2/ai-gateway. The swagger
+// annotations live on the canonical /ai-gateway paths, so the legacy
+// routes have no matching annotation and must be skipped.
+func isLegacyAIBridgeAlias(route string) bool {
+	return strings.HasPrefix(route, aibridge.AIBridgeRootPath+"/")
+}
+
 func VerifySwaggerDefinitions(t *testing.T, router chi.Router, swaggerComments []SwaggerComment, opts ...SwaggerOption) {
 	cfg := swaggerOptions{}
 	for _, opt := range opts {
@@ -206,6 +216,9 @@ func VerifySwaggerDefinitions(t *testing.T, router chi.Router, swaggerComments [
 			if isExperimentalEndpoint(route) {
 				return
 			}
+			if isLegacyAIBridgeAlias(route) {
+				return
+			}
 
 			c := findSwaggerCommentByMethodAndRoute(swaggerComments, method, route)
 			assert.NotNil(t, c, "Missing @Router annotation")