Stream tool execution progress to connected clients

[deathbyknowledge]

Problem

Connected clients currently do not receive per-tool execution progress for session-initiated tools in real time. They typically see only session-level chat events (partial / final) after the agent loop resumes.

Impact

For workflows with long-running tools (especially node/native async tools like future window/render flows), the user has no immediate visibility into:

• tool invocation started
• tool in progress
• intermediate tool results/errors
• completion state until final assistant turn

Current behavior reference

• gateway routes client-driven tool.invoke via pending route kind: "client" and can respond directly with tool.result.
• session-driven tool calls (tool.request) use kind: "session" and feed result into session.toolResult, then LLM continuation.
• chat stream comes from Session.broadcastToClients states.

Request

Add explicit streaming of tool lifecycle events for session sessions, e.g.:

1. Emit tool.started when tool call is dispatched.
2. Emit tool.result / tool.error updates.
3. Optionally emit aggregate tool.phase events for queued/parallel tool batches.
4. Keep tool.request path unchanged for internal logic so behavior remains backward compatible.

Notes

This is intentionally separate from protocol routing/pending-op refactor work currently in progress.