Keep JSON logs when enrichment contains non-serializable values

[groupthinking]

Ensure _format_json never drops a log record when JSON serialization fails, falling back to safe fields and recording serialization_error.

[linear-code]

GRV-385

[github-actions]

👋 Thanks for opening this issue! It has been automatically labeled for triage.

[groupthinking]

Duplicate of #1505. That issue is the detailed, reproduced writeup of exactly this ask (_format_json still drops/corrupts records in specific cases even after the #1491 fallback), with acceptance criteria and regression-test guidance. Closing here to consolidate.

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[groupthinking]

Duplicate of #1499. This is a generic restatement of "never drop a record on non-serializable enrichment" — #1499 covers the same goal with concrete reproductions and acceptance criteria for the current main. Closing to consolidate.

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