fix: resolve test race conditions and update golangci-lint action

- Fix race condition in gateway tests by using TestMain to set Gin mode once
  - Remove duplicate gin.SetMode(gin.TestMode) calls from individual tests
  - Add TestMain function to initialize test environment before all tests
  - Prevents race conditions when tests run in parallel with -race flag

- Update golangci-lint-action from v6 to v7
  - v6 doesn't support golangci-lint v2.x versions
  - v7 supports golangci-lint v2.x and automatically selects compatible version
  - Change version from v2.6.0 to latest for automatic compatibility

All tests now pass with race detector enabled.

.github/workflows/ci.yml

@@ -79,9 +79,9 @@ jobs:
           go-version: '1.25.3'
 
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v6
+        uses: golangci/golangci-lint-action@v7
         with:
-          version: v2.6.0
+          version: latest
           args: --timeout=5m
 
   build: