[fix](cloud) Fix SchemaVariablesScanner crash due to incomplete FE response (#65994)

[deardeng]

pick from #65994
Problem Summary: When the Thrift connection to FE is unstable, a partially deserialized TShowVariableResult can contain variable rows with fewer than four fields. SchemaVariablesScanner accessed fields 0 through 3 unconditionally and could crash the BE process. Validate every row before accessing its fields and return an InternalError asking the user to retry when the response is incomplete.

Release note

Fix a BE crash when scanning variables after receiving an incomplete FE response; the query now returns an error asking the user to retry.

(cherry picked from commit d3bf6b8)

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

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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
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[hello-stephen]

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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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[deardeng]

run buildall

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 44.44% (4/9) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.17% (29234/39956)
Line Coverage	56.89% (319326/561257)
Region Coverage	54.05% (267072/494111)
Branch Coverage	54.87% (118000/215073)