[crashes] Update aggregator pipeline for changestream

[widatama]

No description provided.

[Copilot]

Pull request overview

This PR updates the MongoDB changestream aggregation pipelines in the crashes plugin to properly handle the _id field and ensure consistent field projection across all three event aggregators (crash, crash-session-start, and crash-session-update).

Key Changes:

• Renamed _id to __id in changestream $project stages to avoid field name conflicts when projecting from fullDocument._id
• Added missing fields (uid, up, up_extra) to ensure all necessary data is projected consistently
• Updated lrid assignments to use fallback logic currEvent.__id || currEvent._id to support both changestream and fallback modes

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