feat: achieve rich tool call display parity for agent stream

This change ensures that tool execution in the Agent Protocol experiment
reaches visual parity with the legacy implementation by passing rich
metadata and objects (like FileDiff) through the AgentEvent stream.

Key changes:
- Core 'LegacyAgentSession' now builds temporary invocations for tool requests
  to capture rich, argument-aware descriptions (e.g. 'Writing to poem.md').
- Core 'LegacyAgentSession' now attaches the raw 'resultDisplay' object and
  'outputFile' path to the 'tool_response' event metadata.
- Core 'LegacyAgentSession' now passes the rich description through 'tool_update'
  events to ensure dynamic descriptions are updated during execution.
- UI 'useAgentStream' hook now extracts these rich values from event metadata
  to populate the local 'trackedTools' state, allowing 'mapToDisplay' to
  correctly trigger bespoke rendering components (like the diff viewer).

packages/cli/src/ui/hooks/useAgentStream.ts

@@ -216,6 +216,7 @@ export const useAgentStream = (
               tool: {
                 displayName: (event._meta?.['displayName'] as string) ?? event.name,
                 isOutputMarkdown: (event._meta?.['isOutputMarkdown'] as boolean) ?? false,
+                kind: event._meta?.['kind'] as any,
               },
               invocation: {
                 getDescription: () => (event._meta?.['description'] as string) ?? '',
@@ -234,6 +235,9 @@ export const useAgentStream = (
                     progress: event.data?.['progress'] as number | undefined,
                     progressTotal: event.data?.['progressTotal'] as number | undefined,
                     pid: event.data?.['pid'] as number | undefined,
+                    invocation: {
+                      getDescription: () => (event._meta?.['description'] as string) ?? (tc as any).invocation?.getDescription(),
+                    },
                   } as unknown as TrackedToolCall)
                 : tc,
             ),
@@ -247,7 +251,8 @@ export const useAgentStream = (
                     ...tc,
                     status: event.isError ? 'error' : 'success',
                     response: {
-                      resultDisplay: event.displayContent?.[0]?.type === 'text' ? event.displayContent[0].text : undefined,
+                      resultDisplay: event._meta?.['resultDisplay'] ?? (event.displayContent?.[0]?.type === 'text' ? event.displayContent[0].text : undefined),
+                      outputFile: event._meta?.['outputFile'] as string | undefined,
                     },
                     responseSubmittedToGemini: true,
                   } as unknown as TrackedToolCall)

packages/core/src/agent/legacy-agent-session.ts

@@ -179,6 +179,9 @@ class LegacyAgentProtocol implements AgentProtocol {
                 progressTotal: tc.progressTotal,
                 pid: tc.pid,
               },
+              _meta: {
+                description: tc.invocation.getDescription(),
+              },
             }),
           );
         }
@@ -226,10 +229,12 @@ class LegacyAgentProtocol implements AgentProtocol {
           for (const ev of translatedEvents) {
             if (ev.type === 'tool_request') {
               const tool = this._config.getToolRegistry().getTool(ev.name);
+              const invocation = tool?.build(ev.args);
               ev._meta = {
                 displayName: tool?.displayName ?? ev.name,
-                description: tool?.description ?? '',
+                description: invocation?.getDescription() ?? tool?.description ?? '',
                 isOutputMarkdown: tool?.isOutputMarkdown ?? false,
+                kind: tool?.kind,
               };
             }
           }
@@ -298,6 +303,10 @@ class LegacyAgentProtocol implements AgentProtocol {
               isError: response.error !== undefined,
               ...(displayContent ? { displayContent } : {}),
               ...(data ? { data } : {}),
+              _meta: {
+                resultDisplay: response.resultDisplay,
+                outputFile: response.outputFile,
+              },
             }),
           ]);