[test-improver] Improve tests for tracing package (#5033)

# Test Improvements: `internal/tracing/http_test.go`

## File Analyzed

- **New Test File**: `internal/tracing/http_test.go`
- **Package**: `internal/tracing`
- **Implementation**: `internal/tracing/http.go` —
`statusResponseWriter` wrapper type

## Improvements Made

### 1. New Test Coverage for `statusResponseWriter`

The `statusResponseWriter` type (used inside `WrapHTTPHandler`) had no
dedicated unit tests:

| Method | Before | After |
|--------|--------|-------|
| `Write` | 66.7% | 100% |
| `Unwrap` | **0.0%** | **100%** |
| `WriteHeader` | 100% | 100% |

**Package total: 95.1% → 96.7%**

### 2. Tests Added

- ✅ `TestStatusResponseWriter_WriteHeader` — verifies status code is
captured and forwarded
- ✅ `TestStatusResponseWriter_Write_SetsImplicit200` — verifies implicit
200 set when `Write` called without prior `WriteHeader`
- ✅ `TestStatusResponseWriter_Write_PreservesExplicitStatus` — verifies
an already-set status isn't overwritten by `Write`
- ✅ `TestStatusResponseWriter_Unwrap_ReturnsUnderlying` — verifies
`Unwrap` returns the wrapped `ResponseWriter`
- ✅ `TestStatusResponseWriter_Unwrap_ExposesOptionalInterfaces` —
verifies `http.ResponseController` can discover `http.Flusher` through
`Unwrap` (the primary reason `Unwrap` exists)

### 3. Idiomatic testify usage

Uses `require.NoError` for fatal checks and `assert.Equal`/`assert.True`
for non-fatal assertions, consistent with the rest of the codebase.

## Test Execution

```
--- PASS: TestStatusResponseWriter_WriteHeader (0.00s)
--- PASS: TestStatusResponseWriter_Write_SetsImplicit200 (0.00s)
--- PASS: TestStatusResponseWriter_Write_PreservesExplicitStatus (0.00s)
--- PASS: TestStatusResponseWriter_Unwrap_ReturnsUnderlying (0.00s)
--- PASS: TestStatusResponseWriter_Unwrap_ExposesOptionalInterfaces (0.00s)
ok  	github.com/github/gh-aw-mcpg/internal/tracing	0.018s	coverage: 96.7% of statements
```

## Why This File?

`statusResponseWriter.Unwrap()` existed specifically to allow
`http.ResponseController` to detect optional interfaces on the
underlying writer (e.g. `http.Flusher`), but had 0% coverage — the most
critical path was completely untested. The `Write` method also had a
branch (implicit-200 assignment) that wasn't exercised. These are small,
focused tests that meaningfully verify the contract of the type.

---
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Author: lpcox

internal/tracing/http_test.go

@@ -0,0 +1,73 @@
+package tracing
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// mockFlusher implements http.ResponseWriter and http.Flusher so we can verify
+// that Unwrap gives callers transparent access to the underlying writer's optional
+// interfaces.
+type mockFlusher struct {
+	httptest.ResponseRecorder
+	flushed bool
+}
+
+func (m *mockFlusher) Flush() { m.flushed = true }
+
+func TestStatusResponseWriter_WriteHeader(t *testing.T) {
+	rec := httptest.NewRecorder()
+	srw := &statusResponseWriter{ResponseWriter: rec}
+
+	srw.WriteHeader(http.StatusCreated)
+
+	assert.Equal(t, http.StatusCreated, srw.statusCode)
+	assert.Equal(t, http.StatusCreated, rec.Code)
+}
+
+func TestStatusResponseWriter_Write_SetsImplicit200(t *testing.T) {
+	rec := httptest.NewRecorder()
+	srw := &statusResponseWriter{ResponseWriter: rec}
+
+	// statusCode starts at zero – Write should set it to 200 implicitly.
+	n, err := srw.Write([]byte("hello"))
+	require.NoError(t, err)
+	assert.Equal(t, 5, n)
+	assert.Equal(t, http.StatusOK, srw.statusCode)
+}
+
+func TestStatusResponseWriter_Write_PreservesExplicitStatus(t *testing.T) {
+	rec := httptest.NewRecorder()
+	srw := &statusResponseWriter{ResponseWriter: rec}
+
+	srw.WriteHeader(http.StatusAccepted)
+
+	_, err := srw.Write([]byte("body"))
+	require.NoError(t, err)
+	// statusCode was already set via WriteHeader; Write must not overwrite it.
+	assert.Equal(t, http.StatusAccepted, srw.statusCode)
+	assert.Equal(t, http.StatusAccepted, rec.Code)
+}
+
+func TestStatusResponseWriter_Unwrap_ReturnsUnderlying(t *testing.T) {
+	rec := httptest.NewRecorder()
+	srw := &statusResponseWriter{ResponseWriter: rec}
+
+	underlying := srw.Unwrap()
+	assert.Same(t, rec, underlying, "Unwrap should return the wrapped ResponseWriter")
+}
+
+func TestStatusResponseWriter_Unwrap_ExposesOptionalInterfaces(t *testing.T) {
+	mf := &mockFlusher{ResponseRecorder: *httptest.NewRecorder()}
+	srw := &statusResponseWriter{ResponseWriter: mf}
+
+	// http.ResponseController uses Unwrap to discover optional interfaces like Flusher.
+	rc := http.NewResponseController(srw)
+	err := rc.Flush()
+	require.NoError(t, err, "Flush via ResponseController should succeed when underlying writer is a Flusher")
+	assert.True(t, mf.flushed, "underlying Flusher should have been called through Unwrap")
+}