fix: Preserve metadata in _to_thread_response method

[anshal21]

Problem

The _to_thread_response method is responsible for converting thread objects to response format, filtering out hidden context items, and returning a clean Thread object for client responses. However, it was missing the metadata field in the returned Thread object, causing metadata to be lost in all thread responses sent to clients.

Impact

• ThreadUpdatedEvent would not include metadata set by responders
• Any other code path using _to_thread_response would lose metadata information
• This breaks the documented workflow for preserving context like previous_response_id for Agent SDK runs

Solution

Added the metadata field to the Thread object construction in _to_thread_response:

return Thread(
    id=thread.id,
    title=thread.title,
    created_at=thread.created_at,
    items=items,
    status=thread.status,
    metadata=thread.metadata,  # ← This line was missing
)

Testing

Added a comprehensive test test_metadata_preserved_in_thread_response that:

1. Sets metadata in the responder function
2. Verifies that ThreadCreatedEvent has empty metadata (expected, as it's sent before responder runs)
3. Verifies that ThreadUpdatedEvent correctly includes the metadata (this would fail without the fix)
4. Confirms metadata is properly saved to the store