fix(core): restore structured __repr__ on A2A exceptions

[cchinchilla-dev]

Description

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Fixes #1347 🦕

Problem

#683 added __repr__ methods to the client exception classes (#679). The client error refactor in #761 replaced those classes with the A2AError-based hierarchy, and the __repr__ methods were not carried over. Both domain and client errors now fall back to the default Exception.__repr__, which only wraps str() output and loses structured attributes:

>>> repr(AgentCardResolutionError('Failed to fetch agent card', status_code=404))
"AgentCardResolutionError('Failed to fetch agent card')"  # Loses status_code

>>> repr(InvalidParamsError('Invalid field', data={'field': 'id'}))
"InvalidParamsError('Invalid field')"  # Loses data

Fix

Added __repr__ back, following the same pattern as #683:

A2AError (src/a2a/utils/errors.py): Exposes message and data as keyword arguments. All domain errors (TaskNotFoundError, InvalidParamsError, ...) and client errors (A2AClientError, A2AClientTimeoutError) inherit it.

>>> repr(InvalidParamsError('Invalid field', data={'field': 'id'}))
"InvalidParamsError(message='Invalid field', data={'field': 'id'})"

AgentCardResolutionError (src/a2a/client/errors.py): Exposes status_code, as A2AClientHTTPError did in #683.

>>> repr(AgentCardResolutionError('Failed to fetch agent card', status_code=404))
"AgentCardResolutionError(status_code=404, message='Failed to fetch agent card')"

This change does not alter traceback formatting, __str__ output, exception hierarchy, catching behavior, or any existing error handling logic. It is purely additive and intended to improve debuggability via repr() in REPLs, logs, and structured logging contexts.

Test

Tests were added to the existing suite at tests/client/test_errors.py, and to a new tests/utils/test_a2a_errors.py for A2AError and domain errors (test_errors.py is already taken by the client suite), keeping the same structure and conventions already in place.

No existing test was modified or removed, all additions are purely additive. Happy to adapt the test approach based on maintainer feedback.

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