Fix a2amcp login error reporting

src/oauth.ts

@@ -208,9 +208,10 @@ async function exchangeAuthorizationCode(opts: {
 }
 
 async function parseTokenResponse(resp: Response): Promise<TokenResponse> {
-  const data = (await resp.json().catch(() => ({}))) as TokenResponse;
+  const text = await resp.text();
+  const data = parseJsonObject(text) as TokenResponse;
   if (!resp.ok || !data.access_token) {
-    const message = data.error_description || data.error || resp.statusText;
+    const message = data.error_description || data.error || text.trim() || resp.statusText;
     throw new Error(`Keycloak token exchange failed: ${message}`);
   }
   return data;
@@ -336,3 +337,13 @@ function expiresAt(expiresIn: number | undefined): number | undefined {
 function nowSeconds(): number {
   return Math.floor(Date.now() / 1000);
 }
+
+function parseJsonObject(text: string): Record<string, unknown> {
+  if (!text.trim()) return {};
+  try {
+    const parsed = JSON.parse(text);
+    return parsed && typeof parsed === "object" ? parsed : {};
+  } catch {
+    return {};
+  }
+}