[fix](test) filter joined FEs in sql cache case

[shuke987]

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary:

parse_sql_from_sql_cache.groovy has a testMultiFrontends block that should only run when at least two usable FEs are present.

In fixed-1FE P0 regression environments, SHOW FRONTENDS can still contain another FE row where Alive=true but Join=false. The previous guard only filtered by Alive, so the case could treat that non-joined row as a second FE and run the multi-FE assertions in a single joined FE topology.

This PR filters candidates with both Alive=true and Join=true, so the multi-FE block is skipped unless there are at least two joined FEs.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
    • git diff --check
    • Manual diff review: only the FE candidate filtering in parse_sql_from_sql_cache.groovy changed.
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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4. Which code was refactored and why was this part of the code refactored?
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[shuke987]

run buildall

[shuke987]

run cloud_p0

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.