Add debug logging to deriveAPIFromServerURL

Log entry and each switch-case result in deriveAPIFromServerURL so that
operators can understand which GitHub API URL resolution path was taken
(GitHub.com, GHEC *.ghe.com tenant, or GHES /api/v3 fallback).

Also log the default-URL fallback path in DeriveGitHubAPIURL.

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

Author: github-actions[bot]

internal/envutil/github.go

@@ -70,33 +70,46 @@ func DeriveGitHubAPIURL(defaultURL string) string {
 			return derived
 		}
 	}
-	return strings.TrimRight(strings.TrimSpace(defaultURL), "/")
+	result := strings.TrimRight(strings.TrimSpace(defaultURL), "/")
+	logGitHub.Printf("GitHub API URL falling back to provided default: %s", result)
+	return result
 }
 
 // deriveAPIFromServerURL converts a GITHUB_SERVER_URL to the corresponding API endpoint.
 // GHEC tenants (*.ghe.com): https://tenant.ghe.com → https://copilot-api.tenant.ghe.com
 // GitHub.com: https://github.com → https://api.github.com
 // GHES (all others): https://github.example.com → https://github.example.com/api/v3
 func deriveAPIFromServerURL(serverURL string) string {
+	logGitHub.Printf("Deriving API URL from server URL: %s", serverURL)
+
 	parsed, err := url.Parse(strings.TrimRight(serverURL, "/"))
 	if err != nil || parsed.Host == "" {
+		logGitHub.Printf("Failed to parse server URL or empty host: serverURL=%s, err=%v", serverURL, err)
 		return ""
 	}
 	if parsed.Scheme != "http" && parsed.Scheme != "https" {
+		logGitHub.Printf("Unsupported scheme in server URL: scheme=%s, serverURL=%s", parsed.Scheme, serverURL)
 		return ""
 	}
 
 	hostname := strings.ToLower(parsed.Hostname())
 
 	switch {
 	case hostname == "github.com" || hostname == "www.github.com":
+		logGitHub.Printf("GitHub.com detected, using default API URL: %s", DefaultGitHubAPIBaseURL)
 		return DefaultGitHubAPIBaseURL
 	case strings.HasSuffix(hostname, ".ghe.com"):
+		var apiURL string
 		if port := parsed.Port(); port != "" {
-			return fmt.Sprintf("%s://copilot-api.%s:%s", parsed.Scheme, hostname, port)
+			apiURL = fmt.Sprintf("%s://copilot-api.%s:%s", parsed.Scheme, hostname, port)
+		} else {
+			apiURL = fmt.Sprintf("%s://copilot-api.%s", parsed.Scheme, hostname)
 		}
-		return fmt.Sprintf("%s://copilot-api.%s", parsed.Scheme, hostname)
+		logGitHub.Printf("GHEC tenant detected, using copilot-api subdomain: hostname=%s, apiURL=%s", hostname, apiURL)
+		return apiURL
 	default:
-		return fmt.Sprintf("%s://%s/api/v3", parsed.Scheme, parsed.Host)
+		apiURL := fmt.Sprintf("%s://%s/api/v3", parsed.Scheme, parsed.Host)
+		logGitHub.Printf("GHES instance detected, using /api/v3 path: host=%s, apiURL=%s", parsed.Host, apiURL)
+		return apiURL
 	}
 }