[Refactor] refactor expr explain&toSQL

[Seaven]

Why I'm doing:

What I'm doing:

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.0
  • 3.5
  • 3.4
  • 3.3

[alvin-celerdata]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

---

[github-actions]

[FE Incremental Coverage Report]

❌ fail : 3 / 10 (30.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	com/starrocks/analysis/SubfieldExpr.java	0	1	00.00%	[80]
🔵	com/starrocks/sql/ast/LambdaFunctionExpr.java	0	1	00.00%	[60]
🔵	com/starrocks/analysis/SlotRef.java	0	4	00.00%	[271, 272, 273, 274]
🔵	com/starrocks/analysis/Expr.java	0	1	00.00%	[575]
🔵	com/starrocks/analysis/FunctionCallExpr.java	2	2	100.00%	[]
🔵	com/starrocks/analysis/CastExpr.java	1	1	100.00%	[]

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)