Disallow positional struct casting when field names don’t overlap

[kosiew]

Which issue does this PR close?

• Closes # Eliminate positional fallback in struct casting #19841.

Rationale for this change

Struct-to-struct casting previously fell back to positional mapping when there was no field-name overlap and the number of fields matched. That behavior is ambiguous and can silently produce incorrect results when source/target schemas have different field naming conventions or ordering.

This PR makes struct casting strictly name-based: when there is no overlap in field names between the source and target structs, the cast is rejected with a clear planning error. This prevents accidental, hard-to-detect data corruption and forces callers to provide explicit field names (or align schemas) when casting.

What changes are included in this PR?

• Removed the positional fallback logic for struct casting in cast_struct_column; child fields are now resolved only by name.
• Updated validate_struct_compatibility to error out when there is no field name overlap, instead of allowing positional compatibility checks.
• Updated unit tests to reflect the new behavior (no-overlap casts now fail with an appropriate error).
• Updated SQLLogicTest files to construct structs using explicit field names (e.g. {id: 1} / {a: 1, b: 'x'} or struct(… AS field)), avoiding reliance on positional behavior.
• Improved error messaging to explicitly mention the lack of field name overlap.

Are these changes tested?

Yes.

• Updated existing Rust unit tests in nested_struct.rs to assert the new failure mode and error message.
• Updated SQLLogicTest coverage (struct.slt, joins.slt) to use named struct literals so tests continue to validate struct behavior without positional casting.

Are there any user-facing changes?

Yes — behavioral change / potential breaking change.

• Casting between two STRUCTs with no overlapping field names now fails (previously it could succeed via positional mapping if field counts matched).
• Users must provide explicit field names (e.g. {a: 1, b: 'x'} or struct(expr AS a, expr AS b)) or ensure schemas share field names.
• Error messages are more explicit: casts are rejected when there is “no field name overlap”.

LLM-generated code disclosure

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

[adriangb]

I think we'll need an entry in upgrading.md