feat: lower CASE WHEN in the planner to CaseExpr

[rishitesh-snt]

Rationale for this change

Wires CASE WHEN through the planner so SQL queries can use it. The proof
primitive lands in #2659; this PR only lowers the DataFusion Expr::Case node
onto it.

What changes are included in this PR?

• expr.rs lowers searched (CASE WHEN c THEN v ...) and simple
  (CASE x WHEN v THEN ...) forms to DynProofExpr::try_new_case, and collects
  column references for CASE.
• CASE without ELSE is rejected at planning time (implicit NULL has no
  representation).

Are these changes tested?

Yes. End-to-end prove/verify (case_when_matrix_tests.rs) for numeric,
multi-arm (up to 4 conditions), varchar, nested, and conditional-aggregation
CASE, plus planning-time rejection tests.

Stacked on #2659 (the CaseExpr primitive); review/merge that first.

[codecov]

Codecov Report

❌ Patch coverage is 66.00000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.41%. Comparing base (f0929d8) to head (75e3127).

Files with missing lines	Patch %	Lines
crates/proof-of-sql-planner/src/expr.rs	66.00%	17 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           feat/case-expr    #2661      +/-   ##
==================================================
- Coverage           96.46%   96.41%   -0.06%     
==================================================
  Files                 295      294       -1     
  Lines               52922    52608     -314     
==================================================
- Hits                51053    50723     -330     
- Misses               1869     1885      +16     

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[github-actions]

This PR has been marked as stale due to 30 days of inactivity. It will be closed in 7 days if no further activity occurs.