[Core] Fix user_hash mismatch issue

[DanielZhangQD]

• Fix sky api login skipping SSO when a residual service account token from a previous login is still in config, by clearing it before the first health check.
• Fix ~/.sky/user_hash not being updated after SSO login, causing false "Cluster was created by another user" warnings on subsequent sky launch.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Test sky api login with:
    • basic auth in ingress - local user_hash does not change
    • basic auth in api server - local user_hash is updated
    • auth with service account token - local user_hash is updated
    • auth with SSO - local user_hash is updated
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[gemini-code-assist]

Code Review

This pull request updates the api_login flow to save the endpoint and clear residual service account tokens before the initial health check, ensuring the server correctly triggers SSO when required. It also synchronizes the local user hash with the final authenticated health check response. New unit tests have been added to verify these behaviors. Feedback was provided regarding unused variables in the newly added test cases that should be cleaned up.

[aylei]

LGTM, thanks @DanielZhangQD !