file_search: SSE event emission for progress streaming

[leseb]

Parent

Part of #29

Scope

Add SSE event emission to the file_search filter for client-facing progress indicators.

Events

• file_search_call.in_progress — emitted when the filter begins processing a file_search tool call
• file_search_call.searching — emitted while vector store queries are in flight
• file_search_call.completed — emitted when results are formatted and ready

Dependencies

• Core file_search filter implementation (sibling issue)
• Agentic loop (Filter 7: agentic_loop #26) wired with branch re-entrance
• May require BodyAccess::ReadWrite on response body or a new event injection mechanism in the streaming pipeline