fix: add collision retry for userid generation

tests/test_userid.py

@@ -1,4 +1,9 @@
-from fastapi_oidc_op.userid import generate_userid
+from unittest.mock import AsyncMock
+
+import pytest
+
+from fastapi_oidc_op.models import User
+from fastapi_oidc_op.userid import generate_unique_userid, generate_userid
 
 
 def test_generate_userid_format() -> None:
@@ -18,3 +23,41 @@ def test_generate_userid_uniqueness() -> None:
 def test_generate_userid_is_lowercase() -> None:
     userid = generate_userid()
     assert userid == userid.lower()
+
+
+@pytest.mark.anyio
+async def test_generate_unique_userid_no_collision() -> None:
+    """Returns a userid when no collision occurs."""
+    user_repo = AsyncMock()
+    user_repo.get_by_userid.return_value = None
+
+    result = await generate_unique_userid(user_repo)
+
+    parts = result.split("-")
+    assert len(parts) == 2
+    user_repo.get_by_userid.assert_called_once_with(result)
+
+
+@pytest.mark.anyio
+async def test_generate_unique_userid_retries_on_collision() -> None:
+    """Retries when a collision is detected and succeeds on second attempt."""
+    existing_user = User(userid="taken-xxxxx", username="existing")
+    user_repo = AsyncMock()
+    user_repo.get_by_userid.side_effect = [existing_user, None]
+
+    result = await generate_unique_userid(user_repo)
+
+    assert user_repo.get_by_userid.call_count == 2
+    parts = result.split("-")
+    assert len(parts) == 2
+
+
+@pytest.mark.anyio
+async def test_generate_unique_userid_raises_after_max_retries() -> None:
+    """Raises RuntimeError when max retries are exhausted."""
+    existing_user = User(userid="taken-xxxxx", username="existing")
+    user_repo = AsyncMock()
+    user_repo.get_by_userid.return_value = existing_user
+
+    with pytest.raises(RuntimeError, match="Failed to generate unique userid"):
+        await generate_unique_userid(user_repo)