fix: add GHEC test case and upstream URL to proxy startup output

Agent-Logs-Url: https://github.com/github/gh-aw-mcpg/sessions/610360e8-6143-472a-9f09-bf7a1cf0f2ce

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

internal/cmd/proxy.go

@@ -218,6 +218,7 @@ func runProxy(cmd *cobra.Command, args []string) error {
 		// Print connection info
 		fmt.Fprintf(os.Stderr, "\nMCPG GitHub API Proxy\n")
 		fmt.Fprintf(os.Stderr, "  Listening: %s://%s\n", scheme, actualAddr)
+		fmt.Fprintf(os.Stderr, "  Upstream:  %s\n", apiURL)
 		fmt.Fprintf(os.Stderr, "  Mode:      %s\n", proxyDIFCMode)
 		fmt.Fprintf(os.Stderr, "  Guard:     %s\n", proxyGuardWasm)
 		if tlsCfg != nil {

internal/proxy/proxy_test.go

@@ -933,6 +933,11 @@ func TestDeriveGitHubAPIURL(t *testing.T) {
 			envVars:  map[string]string{"GITHUB_SERVER_URL": "https://github.example.com"},
 			expected: "https://github.example.com/api/v3",
 		},
+		{
+			name:     "GITHUB_SERVER_URL=https://github.com falls back to api.github.com",
+			envVars:  map[string]string{"GITHUB_SERVER_URL": "https://github.com"},
+			expected: DefaultGitHubAPIBase,
+		},
 	}
 
 	for _, tt := range tests {