A test pins signing up with a passkey another account already holds: refused, and no account made #7091

signUpWithPasskey in src/actions/users.ts refuses with passkey_taken, and no test names any of them.

Done:

• [ ] A new test signs up with a passkey, then tries a second sign-up with the same credential id and a different username. It is refused with passkey_taken.
• [ ] No second user row exists, and the first account's passkey is unchanged.
• [ ] The refusal's words do not name the account that holds it.

Files: test/passkey-taken.test.ts

If the test finds the behaviour wrong, fix the action in the same pull and say so. If the check fails in a test this pull does not touch, name it in the pull rather than widening the change.

Before pushing: bun x tsc --noEmit and the new test.