Include populated_from in TransformationError messages

[amc-corey-cox]

Summary

Closes #183

Adds source context to TransformationError so curators can identify which source table/column caused the error without opening the YAML spec.

Before

could not convert string to float: 'A'; class_derivation=Quantity; slot_derivation=value_decimal

After

could not convert string to float: 'A'; class_derivation=Quantity (from pht004041); slot_derivation=value_decimal (from phv00191041)

Changes

• Added class_populated_from and slot_populated_from fields to TransformationError
• Updated __str__ to append (from ...) when populated_from is present
• Pass both fields when constructing the error in map_object

Test plan

• New test: all fields including populated_from in string output
• New test: partial populated_from (only slot, not class)
• Existing tests still pass (10/10)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves transformation diagnostics by including source mapping context (populated_from) in TransformationError string output, helping curators identify the originating source table/column directly from error messages (closes #183).

Changes:

• Added class_populated_from and slot_populated_from fields to TransformationError.
• Updated TransformationError.__str__ to append (from ...) to derivation segments when available.
• Passed populated-from context when constructing TransformationError in ObjectTransformer.map_object.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/linkml_map/transformer/errors.py	Extends TransformationError with populated-from context and formats it into __str__.
src/linkml_map/transformer/object_transformer.py	Supplies class_deriv.populated_from and slot_derivation.populated_from when wrapping exceptions as TransformationError.
tests/test_transformer/test_continue_on_error.py	Adds unit tests covering full and partial populated-from inclusion in TransformationError.__str__.