[log] feat(httputil): add debug logging to HTTP utility functions (#4324)

Add a `logHTTP` debug logger (`httputil:httputil`) and meaningful
logging calls to the three utility functions in
`internal/httputil/httputil.go`.

## Changes

**File modified:** `internal/httputil/httputil.go` (1 file, focused PR)

### Logger added
```go
var logHTTP = logger.New("httputil:httputil")
```

### Logging calls added

| Function | What's logged |
|---|---|
| `WriteJSONResponse` | Status code, serialised body size, and marshal
errors |
| `ParseRateLimitResetHeader` | Parsed reset timestamp (RFC 3339) or
parse failure |
| `IsTransientHTTPError` | Status code when a transient error is
detected |

## Why this is useful

These helpers are called across the `server` and `proxy` packages.
Having debug output here makes it straightforward to trace:
- Which status codes are being written to clients
- Whether `X-RateLimit-Reset` headers are being parsed correctly
(relevant to the circuit-breaker reset logic)
- When the gateway decides a backend response is transient and eligible
for retry

Enable with:
```bash
DEBUG=httputil:* ./awmg --config config.toml
```

## Validation
- `go build ./...` ✅
- `go vet ./internal/httputil/...` ✅
- `go test ./internal/httputil/...` ✅




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Author: lpcox

internal/httputil/httputil.go

@@ -8,18 +8,32 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"github.com/github/gh-aw-mcpg/internal/logger"
 )
 
+var logHTTP = logger.New("httputil:httputil")
+
 // WriteJSONResponse sets the Content-Type header, writes the status code, and encodes
 // body as JSON. It centralises the three-line pattern used across HTTP handlers.
 func WriteJSONResponse(w http.ResponseWriter, statusCode int, body interface{}) {
+	logHTTP.Printf("Writing JSON response: statusCode=%d", statusCode)
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(statusCode)
 	data, err := json.Marshal(body)
 	if err != nil {
+		logHTTP.Printf("Failed to marshal JSON response body: %v", err)
 		return
 	}
-	w.Write(data)
+	logHTTP.Printf("JSON response body size: %d bytes", len(data))
+	n, err := w.Write(data)
+	if err != nil {
+		logHTTP.Printf("Failed to write JSON response body: wrote=%d expected=%d err=%v", n, len(data), err)
+		return
+	}
+	if n != len(data) {
+		logHTTP.Printf("Short write for JSON response body: wrote=%d expected=%d", n, len(data))
+	}
 }
 
 // ParseRateLimitResetHeader parses the Unix-timestamp value of the
@@ -31,15 +45,22 @@ func ParseRateLimitResetHeader(value string) time.Time {
 	}
 	unix, err := strconv.ParseInt(strings.TrimSpace(value), 10, 64)
 	if err != nil {
+		logHTTP.Printf("Failed to parse X-RateLimit-Reset header value=%q: %v", value, err)
 		return time.Time{}
 	}
-	return time.Unix(unix, 0)
+	reset := time.Unix(unix, 0)
+	logHTTP.Printf("Parsed X-RateLimit-Reset: resetAt=%s", reset.UTC().Format(time.RFC3339))
+	return reset
 }
 
 // IsTransientHTTPError returns true for status codes that indicate a temporary
 // server-side condition (rate-limiting or transient failure) worth retrying.
 func IsTransientHTTPError(statusCode int) bool {
-	return statusCode == http.StatusTooManyRequests ||
+	transient := statusCode == http.StatusTooManyRequests ||
 		statusCode == http.StatusServiceUnavailable ||
 		(statusCode >= 500 && statusCode < 600)
+	if transient {
+		logHTTP.Printf("Transient HTTP error detected: statusCode=%d", statusCode)
+	}
+	return transient
 }