test: lock token-bound database routing priority

[FreeOnePlus]

Fixes #64

Root cause

Token-bound database routing is already implemented in the current server, but there was no regression test covering the important case where a valid global Doris account is configured at the same time. That left room for a future regression to silently route an authenticated token through the global high-privilege account.

Change

• Add a regression test with both a valid global account and a token-bound account.
• Assert the dedicated pool is created from the token database configuration.
• Assert the original and active global configurations remain unchanged.

Verification

• uv run pytest --no-cov -q test/utils/test_doris_user_pool_manager.py — 15 passed
• uv run ruff check test/utils/test_doris_user_pool_manager.py — passed
• Full repository tests and coverage run in CI.