Add diagnostic::on_unimplemented attributes to key transmogrify traits

[Copilot]

Compile-time errors on transmogrify are cryptic and unhelpful, showing deeply nested trait bound failures that don't indicate the actual problem (mismatched fields, wrong types, etc.).

Changes

Added #[diagnostic::on_unimplemented] attributes (Rust 1.78+) to key traits used by transmogrify operations:

• Transmogrifier: Shows "Cannot transmogrify {Self} into {Target}" with notes about field compatibility and structure requirements
• ByNameFieldPlucker: Shows "Cannot find field with key {TargetKey} in {Self}" when a required field is missing
• LabelledGeneric: Shows "Cannot derive labelled generic representation for {Self}" with guidance on using the derive macro
• Sculptor: Shows "Cannot sculpt {Self} into the target HList shape" with notes about required types

Also fixed clippy unused imports warning in tuples.rs.

Example

Before:

error[E0277]: the trait bound `HCons<Field<(n, a, m, e), &str>, ...>: 
  Transmogrifier<HCons<Field<(n, a, m, e), &str>, HCons<Field<(e, m, a, i, l), String>, HNil>>, _>` 
  is not satisfied

After:

error[E0277]: Cannot transmogrify `SourceStruct` into `TargetStruct`
  |
  = note: Transmogrify requires that the source and target types have compatible structures.
  = note: The source type must have all the fields needed for the target type, possibly in a different order or nested structure.
  = note: Check that field names match and types are compatible between the source and target.

Original prompt

I want you to work on issue #231

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[Copilot]

Pull request overview

This PR enhances developer experience by adding #[diagnostic::on_unimplemented] attributes to the Transmogrifier and ByNameFieldPlucker traits, transforming cryptic compiler errors into clear, actionable messages.

• Added diagnostic attributes to Transmogrifier trait with helpful conversion error messages
• Added diagnostic attributes to ByNameFieldPlucker trait with field lookup error guidance

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[lloydmeta]

I think there are few more traits that get used by transmogrify, for instance LabelledGeneric is very important.

Also make sure all the CI checks pass.

[Copilot]

I think there are few more traits that get used by transmogrify, for instance LabelledGeneric is very important.

Also make sure all the CI checks pass.

Added diagnostic attributes to LabelledGeneric and Sculptor traits, which are key traits used by transmogrify. Also fixed the clippy warning about unused imports in tuples.rs. Commit: 1be8690

[lloydmeta]

LGTM.

[ExpHP]

Holy moly the future is here