fix(talk): use truncateUtf16Safe for LLM-prompt-facing text truncation (#102477)

* fix(talk): use truncateUtf16Safe for snippet text truncation

fast-context-runtime.ts normalizeSnippet and heartbeat-events-filter.ts
buildExecEventPrompt used raw .slice(0, N), which can produce lone
surrogates when truncation boundaries cross emoji surrogate pairs.

* test(talk): cover UTF-16 snippet boundary

---------

Co-authored-by: hailory <hailory@xydigit.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>
Co-authored-by: Peter Steinberger <peter@steipete.me>

src/talk/fast-context-runtime.test.ts

@@ -79,4 +79,44 @@ describe("resolveRealtimeVoiceFastContextConsult", () => {
     );
     expect(vi.getTimerCount()).toBe(0);
   });
+
+  it("does not split a surrogate pair at the fast-context snippet limit", async () => {
+    const safePrefix = "x".repeat(698);
+    mocks.getActiveMemorySearchManager.mockResolvedValue({
+      manager: {
+        search: vi.fn().mockResolvedValue([
+          {
+            path: "memory/test.md",
+            startLine: 1,
+            endLine: 1,
+            snippet: `${safePrefix}🚀tail`,
+            source: "memory",
+            score: 1,
+          },
+        ]),
+      },
+    });
+
+    const result = await resolveRealtimeVoiceFastContextConsult({
+      cfg: {},
+      agentId: "main",
+      sessionKey: "voice:15550001234",
+      config: {
+        enabled: true,
+        timeoutMs: 1_000,
+        maxResults: 1,
+        sources: ["memory"],
+        fallbackToConsult: true,
+      },
+      args: { question: "What do you remember?" },
+      logger: {},
+    });
+
+    expect(result).toEqual({
+      handled: true,
+      result: {
+        text: expect.stringContaining(`1. [memory] memory/test.md:1-1\n${safePrefix}...`),
+      },
+    });
+  });
 });

src/talk/fast-context-runtime.ts

@@ -6,6 +6,7 @@
  * back to the normal consult flow.
  */
 import { resolveTimerTimeoutMs } from "@openclaw/normalization-core/number-coercion";
+import { truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import type { OpenClawConfig } from "../config/types.openclaw.js";
 import { formatErrorMessage } from "../infra/errors.js";
 import { getActiveMemorySearchManager } from "../plugins/memory-runtime.js";
@@ -69,7 +70,7 @@ function normalizeSnippet(text: string): string {
   }
   // Keep individual memory snippets bounded so several hits still fit in a
   // short realtime response prompt.
-  return `${normalized.slice(0, MAX_SNIPPET_CHARS - 1).trimEnd()}...`;
+  return `${truncateUtf16Safe(normalized, MAX_SNIPPET_CHARS - 1).trimEnd()}...`;
 }
 
 function buildSearchQuery(args: unknown): string {