Confirm that test infrastructure in helpers_test.go meets all acceptance criteria:
✅ MockUpstream and CountingMockServer for httptest-based mock upstreams
✅ Request builders (CreateProxyRequest, CreateMessagesRequest, etc.)
✅ Response assertions (AssertStatusCode, AssertJSONBody, etc.)
✅ Test-mode configuration (ConfigureTestEnv, GetTestMaxRetries)
✅ TestHelperFunctions and TestMockUpstreamBasic tests pass
✅ All tests use mock servers, never hit real api.z.ai
The infrastructure was already implemented. This commit validates it works correctly.
Co-Authored-By: Claude <noreply@anthropic.com>
