Add get_task_tracebacks method to expose stored tracebacks for Tasks

[dotsdl]

Summary

Addresses #347 - Expose ability to get stored tracebacks associated with a given Task.

This PR adds a new method get_task_tracebacks() at all layers of the alchemiscale stack:

• Statestore: Neo4jStore.get_task_tracebacks(task: ScopedKey) - queries Neo4j for Tracebacks nodes associated with failed ProtocolDAGResultRefs for the given Task
• API: GET /tasks/{task_scoped_key}/tracebacks - new endpoint that returns traceback data
• Client: AlchemiscaleClient.get_task_tracebacks(task: ScopedKey) - convenient client method for users

Return Format

Returns a list of dictionaries, one per failed ProtocolDAGResultRef associated with the Task. Each dictionary maps ProtocolUnitFailure GufeKey strings to their corresponding traceback strings:

[
    {
        "ProtocolUnitFailure-ABC123": "Traceback (most recent call last):\n  ...",
        "ProtocolUnitFailure-DEF456": "Traceback (most recent call last):\n  ..."
    },
    # ... more entries if task has multiple failed results
]

Results are ordered by ProtocolDAGResultRef.datetime_created in descending order (newest first).

Example Usage

>>> tracebacks = client.get_task_tracebacks(task_sk)
>>> for tb_dict in tracebacks:
...     for failure_key, traceback_str in tb_dict.items():
...         print(f"Failure {failure_key}: {traceback_str[:100]}...")

Test plan

• Unit tests for statestore method (test_get_task_tracebacks, test_get_task_tracebacks_no_failures, test_get_task_tracebacks_multiple_failures)
• Integration tests for client method (test_get_task_tracebacks, test_get_task_tracebacks_no_failures)
• CI tests pass

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