Fix error handling in dspy parallel

[TomeHirata]

Fixes #8857

[Copilot]

Pull Request Overview

This pull request improves error handling in dspy's parallel execution system by properly tracking failed tasks and their associated exceptions. The changes ensure that when parallel processing encounters errors, the system can provide detailed information about which specific tasks failed and what exceptions occurred.

• Enhanced ParallelExecutor to track failed indices and store exceptions in a mapping structure
• Modified exception handling to return exception markers instead of None, allowing downstream components to distinguish between successful None results and failed executions
• Updated the parallel prediction module to properly populate failed examples and exceptions from the executor's tracking data

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
dspy/utils/parallelizer.py	Added exception tracking infrastructure and modified error handling to preserve exception information
dspy/predict/parallel.py	Enhanced batch processing to extract and return failed examples and exceptions from the executor
tests/utils/test_parallelizer.py	Added comprehensive test for verifying exception tracking functionality
tests/predict/test_parallel.py	Added test to validate batch processing with failed examples

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[isaacbmiller]

Current implementation looks good but misses the error reported in the initial issue of model_dump being called on a None result.

I am not yet familiar with how SubagentTool works