Misc improvements to ProjectionExprs

[adriangb]

Summary

This PR enhances the physical-expr projection handling with several improvements needed for better projection management in datasources.

Changes

1. Add trait implementations:

   • Added PartialEq and Eq for ProjectionExpr
   • Added PartialEq and Eq for ProjectionExprs

2. Add project_batch() method:

   • Efficiently projects RecordBatch with pre-computed schema
   • Handles empty projections correctly
   • Reduces schema projection overhead for repeated calls

3. Fix update_expr() bug:

   • Bug: Previously returned None for literal expressions (no column references)
   • Fix: Now returns Some(expr) for both Unchanged and RewrittenValid states
   • Impact: Critical for queries like SELECT 1 FROM table where no file columns are needed

4. Change from_indices() signature:

   • Changed from &SchemaRef to &Schema for consistency

5. Add comprehensive tests:

   • test_merge_empty_projection_with_literal() - Reproduces roundtrip issue
   • test_update_expr_with_literal() - Tests literal handling
   • test_update_expr_with_complex_literal_expr() - Tests mixed expressions

Part of

This PR is part of #18627 - a larger effort to refactor projection handling in DataFusion.

Testing

All tests pass:

• ✅ New projection tests
• ✅ Existing physical-expr test suite
• ✅ Doc tests