fix: responses api streaming mocks

[supreme-gg-gg]

Implements the correct event lifecycle and schema for Responses API. The previous version works for some simple clients, but is rejected by a strict stateful consumer like Codex (which Kagent runs e2e test on).

Specific problems fixed:

• response.created omitted the required response.
• Text deltas arrived before response.output_item.added and response.content_part.added
• Text events omitted item_id, output_index, and content_index.
• Function-call deltas used arguments instead of delta.
• Function-call events omitted item IDs and output indices.
• It emitted nonexistent response.function_call_output.delta/done events.
• It omitted item/content completion events.
• It split text by bytes, potentially corrupting UTF-8.
• SSE frames lacked the event: <type> line.