fix(plan): preserve decimal scale casts in case and iff

[ck89119]

What type of PR is this?

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

Which issue(s) this PR fixes:

issue #24565

What this PR does / why we need it:

This PR fixes a CASE/IFF type-binding bug for mixed DECIMAL scales.

Before this change, CASE WHEN and IFF could treat DECIMAL branches with the same OID but different scale/width as already matched, so no implicit cast was inserted. In the reported case, the THEN branch stayed at DECIMAL(23,2) while the ELSE branch produced a multiplication result with scale 7, and the final CASE result was formatted with the wrong scale.

This change makes CASE/IFF require casts whenever branch types are not exactly equal to the computed common type, including DECIMAL scale/width differences. It also preserves the maximum DECIMAL scale and width from source branch types when deriving the common return type.

Added coverage:

• focused unit tests for caseCheck and iffCheck
• binder test covering mixed DECIMAL scales in CASE
• BVT reproduction in test/distributed/cases/expression/case_when.sql

Validation:

• mo-tester generate/update result for case_when.sql
• mo-tester run for case_when.sql: 87/87 passed
• go test ./pkg/sql/plan/function -count=1
• go test ./pkg/sql/plan -count=1
• git diff --check
• make
• make static-check

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[ck89119]

Addressed the review concerns in follow-up commit a22f3ab3b.

Changes:

• Restored setMaxScaleFromSource to scale-only behavior.
• Computed a safe common DECIMAL width/scale for CASE/IFF with max(width-scale) + max(scale).
• Narrowed metadata-based implicit cast injection back to DECIMAL only, so same-OID non-DECIMAL types are not pulled into new cast paths.
• Strengthened unit coverage with width assertions.
• Added SQL-level regressions for CASE THEN cast path and IFF mixed DECIMAL scales.

Validation rerun locally:

go test ./pkg/sql/plan/function -run Test_(CaseCheck|IffCheck)_DifferentDecimalScale -count=1
go test ./pkg/sql/plan -run TestBindFuncExprImplByPlanExpr_CaseDifferentDecimalScale -count=1

Prior full checks on this patch set also passed: go test ./pkg/sql/plan/function -count=1, go test ./pkg/sql/plan -count=1, BVT case_when.sql, make, and make static-check.

Note: the temporary pkg/sql/colexec/table_clone/table_clone.go net diff mentioned in this follow-up was removed from the PR scope in commit 3fc5af2b0.

[XuPeng-SH]

Reviewed from multiple angles: the CASE/IFF decimal metadata fix is sound, the decimal width/scale promotion logic is consistent, targeted Go tests with -race passed, and the actual diff matches the intended scope. Approving.