feat: support Z.AI tool_stream for real-time tool call streaming

Add support for Z.AI's native tool_stream parameter to enable real-time
visibility into model reasoning and tool call execution.

- Automatically inject tool_stream=true for zai/z-ai providers
- Allow disabling via params.tool_stream: false in model config
- Follows existing pattern of OpenRouter and OpenAI wrappers

This enables Z.AI API features described in:
https://docs.z.ai/api-reference#streaming

AI-assisted: Claude (OpenClaw agent) helped write this implementation.
Testing: lightly tested (code review + pattern matching existing wrappers)

Closes #18135

src/agents/pi-embedded-runner/extra-params.ts

@@ -172,6 +172,39 @@ function createOpenRouterHeadersWrapper(baseStreamFn: StreamFn | undefined): Str
     });
 }
 
+/**
+ * Create a streamFn wrapper that injects tool_stream=true for Z.AI providers.
+ *
+ * Z.AI's API supports the `tool_stream` parameter to enable real-time streaming
+ * of tool call arguments and reasoning content. When enabled, the API returns
+ * progressive tool_call deltas, allowing users to see tool execution in real-time.
+ *
+ * @see https://docs.z.ai/api-reference#streaming
+ */
+function createZaiToolStreamWrapper(
+  baseStreamFn: StreamFn | undefined,
+  enabled: boolean,
+): StreamFn {
+  const underlying = baseStreamFn ?? streamSimple;
+  return (model, context, options) => {
+    if (!enabled) {
+      return underlying(model, context, options);
+    }
+
+    const originalOnPayload = options?.onPayload;
+    return underlying(model, context, {
+      ...options,
+      onPayload: (payload) => {
+        if (payload && typeof payload === "object") {
+          // Inject tool_stream: true for Z.AI API
+          (payload as Record<string, unknown>).tool_stream = true;
+        }
+        originalOnPayload?.(payload);
+      },
+    });
+  };
+}
+
 /**
  * Apply extra params (like temperature) to an agent's streamFn.
  * Also adds OpenRouter app attribution headers when using the OpenRouter provider.
@@ -209,6 +242,16 @@ export function applyExtraParamsToAgent(
     agent.streamFn = createOpenRouterHeadersWrapper(agent.streamFn);
   }
 
+  // Enable Z.AI tool_stream for real-time tool call streaming.
+  // Enabled by default for Z.AI provider, can be disabled via params.tool_stream: false
+  if (provider === "zai" || provider === "z-ai") {
+    const toolStreamEnabled = merged?.tool_stream !== false;
+    if (toolStreamEnabled) {
+      log.debug(`enabling Z.AI tool_stream for ${provider}/${modelId}`);
+      agent.streamFn = createZaiToolStreamWrapper(agent.streamFn, true);
+    }
+  }
+
   // Work around upstream pi-ai hardcoding `store: false` for Responses API.
   // Force `store=true` for direct OpenAI/OpenAI Codex providers so multi-turn
   // server-side conversation state is preserved.