Fix JSON serialization of contexts in external postprocessors

[arnaugrispg]

Summary

Fixes the external postprocessor when context: true is configured.

memory.contexts contains Pydantic Context instances. Passing them directly to HTTPX through json= caused request construction to fail with:

TypeError: Object of type Context is not JSON serializable

Each context is now converted to a JSON-compatible dictionary using model_dump(mode="json") before building the external request.

Testing

• Added a regression test using a real Context instance.
• Confirmed the test reproduced the serialization error before the fix.
• Verified that the request body is valid JSON after the fix.
• Ran the complete external-agent test file.