feat(httputil): add debug logging to HTTP utility functions

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

- WriteJSONResponse: log status code, body size, and marshal errors
- ParseRateLimitResetHeader: log parsed reset time or parse failures
- IsTransientHTTPError: log when a transient error code is detected

These helpers are called widely across the server and proxy packages,
so debug logging here makes it easier to trace HTTP response behaviour
and rate-limit handling during troubleshooting.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

internal/httputil/httputil.go

@@ -8,17 +8,24 @@ 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
 	}
+	logHTTP.Printf("JSON response body size: %d bytes", len(data))
 	w.Write(data)
 }
 
@@ -31,15 +38,22 @@ 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
 }