Fix struct casts to align fields by name (prevent positional mis-casts)

[kosiew]

Which issue does this PR close?

• Closes Wrong cast between structs with different field order #17285.

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Rationale for this change

Casting between STRUCT types in DataFusion previously relied on physical field order. When a struct literal or expression had the same fields but in a different order than the target schema, DataFusion could silently assign values to the wrong fields (positional matching), causing incorrect results and potential data corruption.

This PR changes struct-to-struct casting behavior to match and reorder fields by name, ensuring that:

• {b: 3, a: 4}::STRUCT(a INT, b INT) yields {a: 4, b: 3}
• nested structs are handled consistently
• missing fields are filled with nulls and extra fields are ignored (where supported by the runtime cast)

This addresses the bug described in #17285 (and aligns with the discussion in #14396 / #17281).

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What changes are included in this PR?

• Name-based struct casting at runtime

  • Added cast_struct_array_by_name in datafusion_common::nested_struct as a small wrapper around existing struct casting logic.
  • Updated ScalarValue::cast_to_with_options to use name-based struct casting when both source and target are structs.
  • Updated ColumnarValue::cast_to (array path) to route struct casts through name-based matching/reordering and fall back to Arrow casting for non-struct types.

• Struct type coercion improvements for binary expressions

  • Updated struct_coercion to attempt name-based field alignment first (when there is name overlap), and to fallback to positional coercion when names don’t match (preserves backward compatibility for unnamed/positional patterns).

• Planner / physical cast permissiveness for struct-to-struct

  • Updated ExprSchemable cast checks to allow struct-to-struct casts during planning even when Arrow’s can_cast_types would reject them, deferring detailed matching to runtime.
  • Updated physical cast_with_options to similarly allow struct-to-struct casts (including field-count mismatches) so execution can apply name-based casting.

• Optimizer safety: avoid const-folding problematic struct casts

  • Added a guard in simplify_expressions to skip const-folding struct casts when field counts mismatch, preventing potential optimizer hangs.

• Tests and sqllogictest coverage

  • Added unit tests validating:

    • field reordering matches by name
    • missing target fields produce nulls

  • Updated/extended SQL logic tests (struct.slt, case.slt) to cover:

    • out-of-order struct literals now working
    • arrays of structs with different field order
    • nested struct reordering
    • casts with missing/extra fields
    • clarified CASE coercion expectations with name-based behavior

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Are these changes tested?

Yes.

• Added Rust unit tests in datafusion/expr-common/src/columnar_value.rs:

  • cast_struct_by_field_name
  • cast_struct_missing_field_inserts_nulls

• Updated and expanded sqllogictest files:

  • datafusion/sqllogictest/test_files/struct.slt
  • datafusion/sqllogictest/test_files/case.slt

These tests cover field reordering, missing/extra fields, nested structs, and ensure behavior matches expectations across planner + execution.

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Are there any user-facing changes?

Yes.

• Behavior change: Struct-to-struct casts now match fields by name rather than position. This prevents silent mis-assignment when schemas differ only by field order.
• Struct literals and arrays of structs with fields in different orders that previously errored (or produced incorrect results) may now succeed.

Potential considerations:

• Users relying on positional behavior when field names don’t align may observe changes; coercion falls back to positional matching only when there is no name overlap.

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LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed and tested.