fix: Handle nested DuckDB TIMESTAMPDIFF rewrites

[ferenc-csaky]

Fixes a DuckDB TIMESTAMPDIFF translation failure where Calcite REINTERPRET nodes could leak into SQL unparsing.

Cause

• SimpleCallTransform stopped traversal whenever it saw a matching operator, even if the transform returned the original call unchanged.
• For nested expressions like CAST, CASE, and NULLIF around TIMESTAMPDIFF(MONTH, ...), this could prevent the DuckDB timestamp diff rewrite from reaching the inner CAST(Reinterpret(MINUS_DATE(...))).
• The unrewritten REINTERPRET then failed during SQL generation with SqlSyntax$7: SPECIAL.

Fix

• Changed SimpleCallTransform to continue visiting child calls when a transform returns the original RexCall.
• This lets the existing TIMESTAMPDIFF rewrite reach nested timestamp-diff decomposition patterns.
  Test coverage:
• Added DuckDB translation coverage for CAST(TIMESTAMPDIFF(MONTH, ...) AS DECIMAL(...)).
• Added coverage for TIMESTAMPDIFF(MONTH, ...) inside CASE WHEN ... / NULLIF(..., 0), matching the failing query shape.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.02%. Comparing base (cc63718) to head (8e4e586).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../datasqrl/calcite/convert/SimpleCallTransform.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2097      +/-   ##
============================================
- Coverage     14.02%   14.02%   -0.01%     
  Complexity      859      859              
============================================
  Files           605      605              
  Lines         17269    17271       +2     
  Branches       2087     2088       +1     
============================================
  Hits           2422     2422              
- Misses        14614    14616       +2     
  Partials        233      233              

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