feat(): Added error stack information for debugging purpose

[benflexcompute]

Note

Low Risk
Only affects the shape/content of generic validation errors by adding an optional debug field; core validation logic is unchanged, with minimal risk limited to downstream consumers assuming a fixed error schema.

Overview
Adds optional stack-trace capture for unexpected exceptions during services.validate_model(): on non-pydantic errors it now records traceback.format_exc() and attaches it to the returned error entry under a new debug field.

Introduces _sanitize_stack_trace() to strip the standard traceback header and redact absolute paths down to the flow360/ subtree (supporting POSIX and Windows paths), and adds unit tests covering sanitization behavior and mixed-path cases.

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