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_test.go

@@ -81,7 +81,7 @@ func TestAIGatewayKeys(t *testing.T) {
 		require.NoError(t, err)
 		fullKey := created.Key
 
-		resp, err := ownerClient.Request(ctx, http.MethodGet, "/api/v2/aibridge/keys", nil)
+		resp, err := ownerClient.Request(ctx, http.MethodGet, "/api/v2/ai-gateway/keys", nil)
 		require.NoError(t, err)
 		t.Cleanup(func() { _ = resp.Body.Close() })
 		require.Equal(t, http.StatusOK, resp.StatusCode)
@@ -137,7 +137,7 @@ func TestAIGatewayKeys(t *testing.T) {
 
 		// Invalid UUID -> 400 (raw request; SDK method accepts uuid.UUID).
 		//nolint:gocritic // Managing AI Gateway keys is owner-only.
-		resp, err := ownerClient.Request(ctx, http.MethodDelete, "/api/v2/aibridge/keys/not-a-uuid", nil)
+		resp, err := ownerClient.Request(ctx, http.MethodDelete, "/api/v2/ai-gateway/keys/not-a-uuid", nil)
 		require.NoError(t, err)
 		t.Cleanup(func() { _ = resp.Body.Close() })
 		require.Equal(t, http.StatusBadRequest, resp.StatusCode)
@@ -148,7 +148,7 @@ func TestAIGatewayKeys(t *testing.T) {
 		})
 		require.NoError(t, err)
 		// SDK returns no code on success, using raw request to check for 204.
-		delResp, err := ownerClient.Request(ctx, http.MethodDelete, "/api/v2/aibridge/keys/"+created.ID.String(), nil)
+		delResp, err := ownerClient.Request(ctx, http.MethodDelete, "/api/v2/ai-gateway/keys/"+created.ID.String(), nil)
 		require.NoError(t, err)
 		defer delResp.Body.Close()
 		require.Equal(t, http.StatusNoContent, delResp.StatusCode)
@@ -333,7 +333,7 @@ func TestAIGatewayKeysDatabaseErrors(t *testing.T) {
 			name:       "CreateDBError",
 			errStore:   aiGatewayKeyErrorStore{insertErr: dbErr},
 			method:     http.MethodPost,
-			path:       "/api/v2/aibridge/keys",
+			path:       "/api/v2/ai-gateway/keys",
 			body:       codersdk.CreateAIGatewayKeyRequest{Name: "db-err-create"},
 			wantStatus: http.StatusInternalServerError,
 			wantMsg:    "Failed to create key. Please retry.",
@@ -342,7 +342,7 @@ func TestAIGatewayKeysDatabaseErrors(t *testing.T) {
 			name:       "ListDBError",
 			errStore:   aiGatewayKeyErrorStore{listErr: dbErr},
 			method:     http.MethodGet,
-			path:       "/api/v2/aibridge/keys",
+			path:       "/api/v2/ai-gateway/keys",
 			wantStatus: http.StatusInternalServerError,
 			wantMsg:    "Failed to list keys.",
 		},
@@ -350,7 +350,7 @@ func TestAIGatewayKeysDatabaseErrors(t *testing.T) {
 			name:       "DeleteDBError",
 			errStore:   aiGatewayKeyErrorStore{deleteErr: dbErr},
 			method:     http.MethodDelete,
-			path:       "/api/v2/aibridge/keys/" + uuid.New().String(),
+			path:       "/api/v2/ai-gateway/keys/" + uuid.New().String(),
 			wantStatus: http.StatusInternalServerError,
 			wantMsg:    "Failed to delete key.",
 		},