Migrate to Pydantic v2: Replace .dict() with .model_dump()

[coderabbitai]

Context

The codebase currently uses Pydantic v1's .dict() method for model serialization. When migrating to Pydantic v2, this will need to be updated to .model_dump() to avoid deprecation warnings.

Background

This was identified during code review of PR #1130, where .dict() is used in the NATS queue implementation for serializing PipelineProcessingTask objects.

Task

When upgrading to Pydantic v2:

• Replace all instances of .dict() with .model_dump() across the codebase
• Test to ensure serialization behavior remains consistent

References

• PR: PSv2: Use connection pooling and retries for NATS #1130
• Comment: PSv2: Use connection pooling and retries for NATS #1130 (comment)
• Requested by: @mihow