fix: extract interaction_id from SSE events for Interactions API chaining

[enjoykumawat]

Summary

• Fix interaction_id extraction from SSE streaming events so function calling works with StreamingMode.SSE + Interactions API
• SSE events carry the ID in event.interaction.id (start/complete events) or event.interaction_id (status_update), not event.id as the code assumed

Root Cause

call_interactions_api() checked event.id to extract the interaction ID during SSE streaming, but SSE event types (InteractionStartEvent, InteractionStatusUpdate, ContentDelta, etc.) don't have a top-level id attribute:

SSE Event Type	Has id?	Has interaction_id?	Has interaction.id?
ContentDelta	No	No	No
ContentStop	No	No	No
InteractionStatusUpdate	No	Yes	No
InteractionStartEvent	No	No	Yes
InteractionCompleteEvent	No	No	Yes

As a result, current_interaction_id was never set, all session events had interaction_id=None, and _find_previous_interaction_id() could not chain interactions — causing a 400 error on the second LLM call.

Test Plan

• Test InteractionStartEvent ID extraction via event.interaction.id
• Test InteractionStatusUpdate ID extraction via event.interaction_id
• Test ContentDelta correctly returns no ID
• Test interaction_id propagation to status_update LlmResponse
• All existing interaction tests pass

Fixes #5169