Remove collect_with_error and update error handling

[kevinmessiaen]

Remove collect_with_errors and stream_with_errors methods, consolidating error handling into collect and stream with an error_policy parameter and a unified return list for ErrorPolicy.COLLECT (similar to asyncio.gather).

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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