test(cors): write tests for build_cors() function (TDD red)

Add failing test cases for the CORS configuration builder function.
Tests verify correct initialization of CorsSettings with allowed origins,
credentials, and max age configuration. These tests fail until build_cors()
is implemented in the green phase.

Ref: T013 (specs/001-modbus-relay-control)

specs/001-modbus-relay-control/tasks.md

@@ -104,13 +104,13 @@
   - **File**: backend/settings/production.yaml
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T013** [Setup] [TDD] Write tests for build_cors() function
-  - Test: build_cors() with wildcard origin creates permissive Cors (allows any origin)
-  - Test: build_cors() with specific origin creates restrictive Cors
-  - Test: build_cors() with `credentials=true` and wildcard origin returns error (browser constraint violation)
-  - Test: build_cors() sets correct methods (GET, POST, PUT, PATCH, DELETE, OPTIONS)
-  - Test: build_cors() sets correct headers (content-type, authorization)
-  - Test: build_cors() sets max_age from settings
+- [x] **T013** [Setup] [TDD] Write tests for build_cors() function
+  - Test: build_cors() with wildcard origin creates permissive Cors (allows any origin) ✓
+  - Test: build_cors() with specific origin creates restrictive Cors ✓
+  - Test: build_cors() with `credentials=true` and wildcard origin returns error (browser constraint violation) ✓
+  - Test: build_cors() sets correct methods (GET, POST, PUT, PATCH, DELETE, OPTIONS) ✓
+  - Test: build_cors() sets correct headers (content-type, authorization) ✓
+  - Test: build_cors() sets max_age from settings ✓
   - **File**: backend/src/startup.rs (in tests module)
   - **Complexity**: Medium | **Uncertainty**: Low