[refactor](Expr) Extract getString* methods to ExprToStringValueVisitor

[morrySnow]

Move getStringValueForQuery, getStringValueInComplexTypeForQuery, and getStringValueForStreamLoad logic from 17 Expr subclasses into a centralized ExprToStringValueVisitor, following the same visitor pattern as ExprToSqlVisitor and ExprToThriftVisitor.

New classes:

• ExprToStringValueVisitor: visitor with visit methods for DateLiteral, FloatLiteral, BoolLiteral, NullLiteral, DecimalLiteral, ArrayLiteral, MapLiteral, StructLiteral, CastExpr, StringLiteral, IPv4Literal, IPv6Literal, VarBinaryLiteral, PlaceHolderExpr, JsonLiteral, MaxLiteral
• StringValueContext: carries FormatOptions and mode flags (forStreamLoad, inComplexType)

Expr base class methods now delegate to the visitor. Subclass overrides are removed. Also fixes pre-existing local variable naming in DateLiteral static block (DATE_KEY_FORMATTER -> dateKeyFmt) and makes convertToString private (no external callers).

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

[Thearas]

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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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