[fix](nereids): quote slot labels in routine-load legacy expr translation to avoid reserved-keyword parse failure

[Baymine]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:
When a routine load job uses a column name that is a SQL reserved keyword (e.g., group) in a PRECEDING FILTER clause, the
Nereids-to-legacy expression translator sets the slot label as the raw name (e.g., group) without quoting. When the legacy
expression SQL is later re-parsed (e.g., during routine load reparse via NereidsLoadUtils.parseExpressionSeq), the unquoted
reserved keyword causes a parse failure, pausing the routine load job.

This PR quotes the slot label using SqlUtils.getIdentSql() so that reserved-keyword column names are properly backtick-quoted in
the translated legacy expression SQL, preventing the parse failure.

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:

  • Yes. Slot labels in routine-load legacy expression translation are now backtick-quoted, which changes the
    toSqlWithoutTbl() output but does not affect semantics.

• Does this need documentation?

  • No.

Check List (For Reviewer who merge this PR)

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

[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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[Baymine]

run buildall