PhysicalExprAdapter - Schema Rewriter Cleanup

[kosiew]

Refactor PhysicalExprAdapter and schema rewriter utilities to improve maintainability, extract helper functions, and update the factory pattern to be fallible. All changes must remain behavior-preserving.

Scope

This PR focuses on cleanup and refactoring only — no behavioral changes.

Tasks

• Refactor schema rewriter utilities for clarity
• Extract reusable helper functions (nullability checks, metadata propagation, etc.)
• Update adapter factory pattern to support fallible initialization
• Consolidate duplicate logic in schema adaptation paths
• Add unit tests to validate refactored code is behavior-preserving

Files/Modules

• datafusion/physical-expr-adapter/src/schema_rewriter.rs
• datafusion/physical-expr-adapter/src/lib.rs
• Related adapter tests

Acceptance Criteria

✅ All existing tests pass without modification
✅ Refactored code is clearer and more maintainable
✅ No change in adapter output or behavior
✅ New helper functions are well-documented