[fix](fe) Stabilize optimize available MV unit test

[seawinde]

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary:
OptimizeGetAvailableMvsTest could fail intermittently because TestWithFeService reuses the same FE environment across test methods. If a materialized view survived a failed assertion, a later case could see an extra rewrite candidate. The partition-prune case also depended on asynchronous MTMV refresh state before the materialized view became a rewrite candidate, so the candidate map could be empty.

Root cause: In OptimizeGetAvailableMvsTest, test methods created materialized views with fixed names and only dropped them at the end of the happy path, so failed assertions could leave state behind for later test methods. The partition-prune case also did not mock MTMV.canBeCandidate(), which made the test depend on asynchronous MTMV refresh state.

Change Summary:

File	Change Description
OptimizeGetAvailableMvsTest.java	Drop test materialized views before and after each test.
OptimizeGetAvailableMvsTest.java	Mock MTMV.canBeCandidate() in the partition-prune 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

• 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?

[seawinde]

run buildall

[hello-stephen]

FE UT Coverage Report

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