fix: add collision retry for userid generation

src/fastapi_oidc_op/userid.py

@@ -2,6 +2,8 @@ import secrets
 
 from proquint import uint2quint
 
+from fastapi_oidc_op.store.protocols import UserRepository
+
 
 def generate_userid() -> str:
     """Generate a unique user identifier in proquint format.
@@ -9,3 +11,17 @@ def generate_userid() -> str:
     Returns a 32-bit proquint string like 'lusab-bansen'.
     """
     return uint2quint(secrets.randbits(32))
+
+
+async def generate_unique_userid(user_repo: UserRepository, max_retries: int = 10) -> str:
+    """Generate a userid that does not already exist in the repository.
+
+    Calls generate_userid() and checks for collisions via user_repo.get_by_userid().
+    Retries up to max_retries times before raising RuntimeError.
+    """
+    for _ in range(max_retries):
+        candidate = generate_userid()
+        existing = await user_repo.get_by_userid(candidate)
+        if existing is None:
+            return candidate
+    raise RuntimeError(f"Failed to generate unique userid after {max_retries} retries")