Run Task Agent

Execute the Task Agent with a user message. The agent will: 1. Process the message using the configured LLM 2. Reason about which task tools to invoke 3. Execute relevant Tasks as needed 4. Return a streaming response Responses are streamed using Server-Sent Events (SSE). The first SSE event emitted after successful stream setup is: - `event_type: "session_id"` with the resolved `session_id` in `metadata` Use `revision_id` to execute a specific TaskAgent revision. New sessions can run any revision belonging to the TaskAgent. Existing sessions remain pinned to their original revision; passing a conflicting `revision_id` is rejected. Subsequent events include tool calls/results, text responses, metadata-only `memory_compaction` events when compaction memory runs, and the terminal `done` or `error` event. Compaction metadata may include token-threshold configuration such as `trigger: "auto"`, `token_threshold`, and `event_retention_size` when token-aware compaction is active.