feat(agent): emit structured _tool_events progress metadata

Extend the existing on_progress callback to carry structured tool-event
payloads alongside the plain-text hint, so channels can render rich
tool execution state (start/finish/error, arguments, results, file
attachments) rather than only the pre-formatted hint string.

Changes
-------
- AgentLoop._tool_event_start_payload() — builds a version-1 start
  payload from a ToolCallRequest
- AgentLoop._tool_event_result_extras() — extracts files/embeds from a
  tool result dict
- AgentLoop._tool_event_finish_payloads() — maps tool_calls +
  tool_results + tool_events from AgentHookContext into finish payloads
- _LoopHook.before_execute_tools() — passes tool_events=[...] to
  on_progress together with the existing tool_hint flag
- _LoopHook.after_iteration() — emits a second on_progress call with
  the finish payloads once tool results are available
- _bus_progress() — forwards tool_events as _tool_events in OutboundMessage
  metadata so channel implementations can read them
- on_progress type widened to Callable[..., Awaitable[None]] on all
  public entry points; _cli_progress updated to accept and ignore
  tool_events

The contract is additive: callers that only accept (content, *, tool_hint)
continue to work unchanged. Callers that also accept tool_events receive
the structured data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/tools/test_message_tool_suppress.py

@@ -152,7 +152,6 @@ class TestMessageToolSuppressLogic:
             ('read foo.txt', True),
         ]
 
-
 class TestMessageToolTurnTracking:
 
     def test_sent_in_turn_tracks_same_target(self) -> None: