docs(bf-3zp): verify test infrastructure for mock upstream server

Verified comprehensive test infrastructure already exists in proxy/helpers_test.go:
- MockUpstream with configurable scenarios
- Test mode configuration (TestModeEnv, TestMaxRetriesEnv)
- Request builders and response assertions
- Proxy handler factory and execution helpers
- All helper tests pass successfully

No code changes needed - infrastructure is complete and functional.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/notes/bf-3zp.md

@@ -0,0 +1,91 @@
+# Test Infrastructure Verification (bf-3zp)
+
+## Summary
+
+Verified that comprehensive test infrastructure for mock upstream server already exists in `proxy/helpers_test.go`.
+
+## Existing Infrastructure
+
+### 1. MockUpstream Server
+- `MockUpstream` struct with httptest.Server
+- Configurable scenarios (429, empty body, invalid JSON, etc.)
+- Request counting and failure simulation
+- Custom handler support
+
+### 2. Test Mode Configuration
+- `TestModeEnv` environment variable support
+- `TestMaxRetriesEnv` for fast test execution
+- `ConfigureTestEnv()` helper function
+- `IsTestMode()` and `GetTestMaxRetries()` functions
+
+### 3. Test Request Builders
+- `CreateProxyRequest()` - Generic request builder
+- `CreateMessagesRequest()` - /v1/messages requests
+- `CreateStreamingMessagesRequest()` - Streaming requests
+- `CreateNonStreamingMessagesRequest()` - Non-streaming requests
+
+### 4. Response Assertions
+- `AssertStatusCode()` - Status code validation
+- `AssertJSONBody()` - JSON validation
+- `AssertResponseField()` - Field value checking
+- `AssertEmptyBody()` - Empty body validation
+- `AssertHeader()` - Header validation
+- `AssertContentType()` - Content-Type validation
+
+### 5. Proxy Handler Factory
+- `CreateTestProxyHandler()` - Creates configured ProxyHandler instances
+- Automatically sets test environment variables
+- Configures high rate limits to avoid test interference
+
+### 6. Test Execution Helpers
+- `ExecuteProxyRequest()` - Execute request through proxy
+- `ExecuteMessagesRequest()` - Execute /v1/messages request
+- Response reading helpers (`ReadResponseBody`, `ReadJSONResponse`)
+
+### 7. Duration Assertions
+- `AssertDurationAtLeast()` - Minimum duration validation
+- `AssertDurationInRange()` - Range validation
+- `CalculateBackoffDelay()` - Backoff calculation helper
+- `CalculateTotalMaxDelay()` - Total delay calculation
+
+### 8. Mock Server Helpers
+- `CreateCountingMockServer()` - Counting mock server
+- `CountingMockServer` - Wraps httptest.Server with counting
+
+### 9. Additional Helpers
+- `WaitForCondition()` - Polling helper
+- `AssertUpstreamCallCount()` - Request count validation
+- `ResetMetrics()` - Metrics reset helper
+- `CreateTestRequestBody()` - Body builder
+
+## Verification Results
+
+All helper tests pass:
+```bash
+go test ./proxy/ -run TestHelper -v
+=== RUN   TestHelperFunctions
+--- PASS: TestHelperFunctions (0.00s)
+    --- PASS: TestHelperFunctions/IsTestMode (0.00s)
+    --- PASS: TestHelperFunctions/GetTestMaxRetries (0.00s)
+    --- PASS: TestHelperFunctions/CalculateBackoffDelay (0.00s)
+    --- PASS: TestHelperFunctions/CreateNonStreamingMessagesRequest (0.00s)
+    --- PASS: TestHelperFunctions/CreateStreamingMessagesRequest (0.00s)
+```
+
+Mock upstream tests pass:
+```bash
+go test ./proxy/ -run TestMockUpstreamBasic -v
+=== RUN   TestMockUpstreamBasic
+--- PASS: TestMockUpstreamBasic (0.00s)
+    --- PASS: TestMockUpstreamBasic/success_scenario (0.00s)
+    --- PASS: TestMockUpstreamBasic/429_scenario (0.00s)
+```
+
+## Conclusion
+
+The test infrastructure described in bead bf-3zp already exists and is fully functional. No new code changes are needed. The infrastructure includes:
+- ✅ net/http/httptest-based mock upstream server
+- ✅ Test helper functions for common setup
+- ✅ Environment/test-mode flags for fast execution
+- ✅ Tests never hit real api.z.ai (uses mock servers)
+- ✅ All helper tests pass