fix: prevent default profile credential collision (#1628)

* fix: prevent default profile credential collision

* fix: isolate default profile credential fallback

src/auth/account-context.ts

@@ -36,6 +36,7 @@ export const DEFAULT_ACCOUNT_CONTEXT_GROUP = 'default';
 export const DEFAULT_ACCOUNT_CONTINUITY_MODE: AccountContinuityMode = 'standard';
 export const MAX_CONTEXT_GROUP_LENGTH = 64;
 export const ACCOUNT_PROFILE_NAME_PATTERN = /^[a-zA-Z][a-zA-Z0-9_-]*$/;
+const RESERVED_ACCOUNT_PROFILE_NAMES = new Set(['default']);
 
 const CONTEXT_GROUP_PATTERN = /^[a-zA-Z][a-zA-Z0-9_-]*$/;
 
@@ -57,7 +58,10 @@ export function isValidContextGroupName(value: string): boolean {
  * Validate account profile naming constraints.
  */
 export function isValidAccountProfileName(value: string): boolean {
-  return ACCOUNT_PROFILE_NAME_PATTERN.test(value);
+  return (
+    ACCOUNT_PROFILE_NAME_PATTERN.test(value) &&
+    !RESERVED_ACCOUNT_PROFILE_NAMES.has(value.toLowerCase())
+  );
 }
 
 /**