fix: OAuth token refresh failure returns undefined instead of throwing

When OAuth refresh fails during model discovery, getApiKey() now returns
undefined instead of throwing. This allows the app to start and fall back
to other providers, so the user can /login to re-authenticate.

fixes #498

packages/coding-agent/src/core/auth-storage.ts

@@ -326,7 +326,7 @@ export class AuthStorage {
 					if (result) {
 						return result.apiKey;
 					}
-				} catch (err) {
+				} catch {
 					// Refresh failed - re-read file to check if another instance succeeded
 					this.reload();
 					const updatedCred = this.data[provider];
@@ -339,13 +339,9 @@ export class AuthStorage {
 							: updatedCred.access;
 					}
 
-					// Refresh truly failed - DO NOT remove credentials
-					// User can retry or re-authenticate manually
-					const errorMessage = err instanceof Error ? err.message : String(err);
-					throw new Error(
-						`OAuth token refresh failed for ${provider}: ${errorMessage}. ` +
-							`Please try again or re-authenticate with /login.`,
-					);
+					// Refresh truly failed - return undefined so model discovery skips this provider
+					// User can /login to re-authenticate (credentials preserved for retry)
+					return undefined;
 				}
 			} else {
 				// Token not expired, use current access token