feat: implement null-safe equal operator (<=>)

[gouhongshen]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue ##23009

What this PR does / why we need it:

Summary
This PR implements the NULL-safe equal operator (<=>), providing full compatibility with MySQL semantics. It ensures that NULL <=> NULL returns 1 (TRUE) and Value <=> NULL returns 0 (FALSE), never returning NULL.

Changes

• Function Registration: Added NULL_SAFE_EQUAL to function_id.go and initialized metadata in init.go and list_operator.go.
• Execution Logic: Implemented vectorized nullSafeEqualFn in func_compare.go with support for all major data types (Numeric, String, JSON, Decimal, Date/Time, etc.).
• Plan & Binding:
  • Updated base_binder.go to bind tree.NULL_SAFE_EQUAL to the <=> function.
  • Implemented Tuple expansion logic: (a, b) <=> (c, d) is automatically expanded to (a <=> c) AND (b <=> d).
• Optimizer Integration: Marked the operator as PRODUCE_NO_NULL, allowing the optimizer to deduce NotNullable properties and perform optimizations like IS NULL constant folding.
• Safety & Correctness:
  • Join: Explicitly excluded from equi-join identification (IsEqualFunc) to avoid incorrect result dropping in the current Hash
    Join implementation regarding NULL keys.
  • Zonemap: Disabled ZONEMAPPABLE flag to prevent incorrect block pruning, as the generic evaluator currently lacks specialized logic for NULL-safe comparisons.

Testing

• Added comprehensive BVT cases in test/distributed/cases/function/func_null_safe_equal.sql.
• Coverage:
  • Basic scalar comparisons with all NULL combinations.
  • Table-based filtering and Join correctness (NULL matching NULL).
  • Complex types: JSON, Decimal (with scale alignment), and Date/Time.
  • Tuple comparisons and nested logic.
  • Optimizer attribute verification (verifying results are never NULL).
• All tests verified against a live MatrixOne instance.