fix: fixed json serialization

[hucruz]

What does this PR do?

This PR fixes an issue with the JSON serialization of Session objects in the RedisMemorySessionService. Specifically, it adds a robust serializer that correctly handles non-JSON-native types such as:

• set → converted to list
• bytes → decoded to utf-8 or fallback to repr
• datetime, uuid, and Decimal → stringified
• float("NaN"), float("Infinity") → mapped to safe strings

This improves compatibility with Pydantic deserialization and prevents ValidationError when reading sessions from Redis.

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Why is this important?

Previously, using json.dumps(..., default=str) could result in values like "set()" being stored, which causes failures in Session.from_dict() and Event.model_validate(...). This fix prevents such corrupted data and maintains symmetry between to_dict() and the stored Redis payload.

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How is it implemented?

• A custom _json_serializer() function was added to redis_memory_session_service.py.
• _save_sessions(...) now uses this function via json.dumps(..., default=...).
• No changes were made outside this file to keep the scope isolated and testable.