[draft][test](regression) Refine file cache query limit regression case

[wenzhenghu]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary:

This PR refactors the file cache query limit regression case to make it more stable and easier to maintain. It replaces the incorrect use of the Hive metastore IP with real backend endpoints discovered from show backends, aggregates file cache statistics by backend instead of reading a random single row, replaces fixed sleeps with polling, and tightens the query limit assertion to compare against the expected per-query limit. It also simplifies messages, logs, naming, and comments in the regression case.

Release note

None

Check List (For Author)

• Test
  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • 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

CI will be triggered by a run buildall comment right after PR creation.

• 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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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[wenzhenghu]

run buildall

[wenzhenghu]

/review

[wenzhenghu]

run Doris_External_Regression

[wenzhenghu]

run buildall

[wenzhenghu]

run buildall