feat(extensions): forward message and tool execution events to extensions (#1375)

The extension system currently only forwards agent_start, agent_end,
turn_start, and turn_end events. This means extensions cannot access
streaming text (token-by-token), message lifecycle, or tool execution
progress — all of which are available to internal subscribers.

This adds forwarding for the remaining 6 agent event types:
- message_start, message_update, message_end
- tool_execution_start, tool_execution_update, tool_execution_end

These follow the exact same pattern as the existing forwarded events:
new interfaces in types.ts, exports in index.ts, and else-if blocks
in _emitExtensionEvent(). The new types are included in ExtensionEvent
and automatically flow through RunnerEmitEvent (they're not in the
exclusion list).

This enables extensions to build real-time UIs, streaming WebSocket
bridges, and other integrations that need fine-grained event access.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

packages/coding-agent/src/core/extensions/index.ts

@@ -81,9 +81,13 @@ export type {
 	LoadExtensionsResult,
 	LsToolCallEvent,
 	LsToolResultEvent,
+	// Events - Message
+	MessageEndEvent,
 	// Message Rendering
 	MessageRenderer,
 	MessageRenderOptions,
+	MessageStartEvent,
+	MessageUpdateEvent,
 	ModelSelectEvent,
 	ModelSelectSource,
 	// Provider Registration
@@ -124,6 +128,10 @@ export type {
 	ToolCallEventResult,
 	// Tools
 	ToolDefinition,
+	// Events - Tool Execution
+	ToolExecutionEndEvent,
+	ToolExecutionStartEvent,
+	ToolExecutionUpdateEvent,
 	ToolInfo,
 	ToolRenderResultOptions,
 	ToolResultEvent,