docs(bf-2kk): verify response validation test coverage

All required tests already exist and pass:
- Empty/invalid JSON body (non-streaming) triggers retries → 502
- Empty streaming response (zero bytes) triggers retries → 502
- Upstream call counts verified (1 + MAX_RETRIES)
- Metric assertions verified (error_type labels)
- go test ./proxy/ -run TestResponseValidation -race PASS

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

notes/bf-2kk.md

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+# Task BF-2KK: Response Validation Unit Tests
+
+## Summary
+
+All unit tests for response validation failures already exist in `proxy/response_validation_test.go` and pass successfully with race detector enabled.
+
+## Test Coverage Verification
+
+### 1. Empty JSON Body (Non-Streaming) ✅
+**Test:** `TestResponseValidation_EmptyJSONNonStreaming`
+- Covers MAX_RETRIES=1, 2, and 5
+- Verifies 502 Bad Gateway returned after exhausting retries
+- Verifies exact upstream call count (1 + MAX_RETRIES)
+- Verifies exponential backoff timing
+
+**Result:** PASS (35.03s with -race)
+
+### 2. Invalid JSON Body (Non-Streaming) ✅
+**Test:** `TestResponseValidation_InvalidJSONNonStreaming`
+- Covers malformed JSON, truncated JSON, and garbage data
+- Verifies 502 Bad Gateway returned after exhausting retries
+- Verifies exact upstream call count (1 + MAX_RETRIES)
+
+**Result:** PASS (11.01s with -race)
+
+### 3. Empty Streaming Response (Zero Bytes) ✅
+**Test:** `TestResponseValidation_EmptyStreaming`
+- Covers MAX_RETRIES=1, 2, and 4
+- Verifies 502 Bad Gateway returned after exhausting retries
+- Verifies exact upstream call count (1 + MAX_RETRIES)
+- Verifies exponential backoff timing
+
+**Result:** PASS (19.02s with -race)
+
+### 4. Streaming Zero-Byte Detection ✅
+**Test:** `TestResponseValidation_StreamingZeroBytesPeek`
+- Tests zero bytes on first peek triggers retry
+- Tests non-zero bytes on first peek passes through
+
+**Result:** PASS (1.00s with -race)
+
+### 5. Metric Assertions ✅
+**Tests:**
+- `TestResponseValidation_MetricsEmptyJSON` - Verifies `error_type="truncated_response"`
+- `TestResponseValidation_MetricsInvalidJSON` - Verifies `error_type="truncated_response"`
+- `TestResponseValidation_MetricsEmptyStreaming` - Verifies `error_type="empty_streaming"`
+
+**Coverage:**
+- `zai_proxy_upstream_errors_total` increments correctly
+- `zai_proxy_retry_attempts_total` increments correctly
+- Upstream call counts verified
+
+**Result:** All PASS (13.02s total with -race)
+
+### 6. Additional Coverage ✅
+**Tests:**
+- `TestResponseValidation_ValidResponsePassesThrough` - Ensures valid responses don't trigger retries
+- `TestResponseValidation_VariousInvalidBodies` - Comprehensive invalid body test matrix
+
+## Acceptance Criteria Status
+
+| Criterion | Status | Notes |
+|-----------|--------|-------|
+| Tests in proxy/response_validation_test.go | ✅ | File exists with comprehensive coverage |
+| Empty/invalid JSON (non-streaming) coverage | ✅ | Multiple test cases with various MAX_RETRIES |
+| Zero-byte streaming coverage | ✅ | Multiple test cases with various MAX_RETRIES |
+| Upstream call count assertions | ✅ | All tests verify `1 + MAX_RETRIES` |
+| Metric assertions for upstream errors | ✅ | Dedicated metric tests verify error_type labels |
+| go test ./proxy/ -run TestResponseValidation -race | ✅ | PASS in 86.118s |
+
+## Test Execution
+
+```bash
+go test ./proxy/ -run TestResponseValidation -v -race
+```
+
+**Output:** All 15 test cases PASS
+
+## Notes
+
+The tests use the test infrastructure from bf-3zp (helpers_test.go), including:
+- `createTestProxyHandler()` - Creates configured ProxyHandler instances
+- `CreateNonStreamingRequestBody()` / `CreateStreamingRequestBody()` - Request builders
+- `ConfigureTestEnv()` - Test environment setup
+- Metric reset and verification utilities
+
+No additional work was required - the tests were already comprehensive and passing.