[Bug] Add json serilization to posthog integration

[shivamrawat1]

Relevant issues

#18332

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Type

🐛 Bug Fix

Changes

Replaced json=payload with content=safe_dumps(payload) — manually serialize using safe_dumps() to handle non-JSON-serializable objects (datetime, sets, circular refs, etc.).
Switched from json= to content= parameter — content= accepts a pre-serialized string, while json= would use httpx's default serializer and fail on non-serializable objects.

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Project	Deployment	Review	Updated (UTC)
litellm	Ready	Preview, Comment	Dec 23, 2025 0:01am

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