fix: stabilize streamed agent text

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/utils/streamingText.ts

@@ -0,0 +1,67 @@
+function tokenize(value: string): string[] {
+  return value
+    .toLowerCase()
+    .split(/[^a-z0-9]+/i)
+    .filter((token) => token.length > 0);
+}
+
+function computeSuffixPrefixOverlap(current: string, incoming: string): number {
+  const maxOverlap = Math.min(current.length, incoming.length);
+  for (let length = maxOverlap; length > 0; length -= 1) {
+    if (current.slice(-length) === incoming.slice(0, length)) {
+      return length;
+    }
+  }
+
+  return 0;
+}
+
+function shouldReplaceWithSnapshot(current: string, incoming: string): boolean {
+  if (incoming.length < Math.floor(current.length * 0.6)) {
+    return false;
+  }
+
+  const currentTokens = new Set(tokenize(current));
+  const incomingTokens = new Set(tokenize(incoming));
+  if (currentTokens.size < 3 || incomingTokens.size < 3) {
+    return false;
+  }
+
+  let shared = 0;
+  for (const token of incomingTokens) {
+    if (currentTokens.has(token)) {
+      shared += 1;
+    }
+  }
+
+  return shared / Math.min(currentTokens.size, incomingTokens.size) >= 0.5;
+}
+
+export function mergeStreamingText(current: string, incoming: string): string {
+  if (!current) {
+    return incoming;
+  }
+
+  if (!incoming) {
+    return current;
+  }
+
+  if (incoming.startsWith(current) || incoming.includes(current)) {
+    return incoming;
+  }
+
+  if (current.includes(incoming)) {
+    return current;
+  }
+
+  const overlap = computeSuffixPrefixOverlap(current, incoming);
+  if (overlap > 0) {
+    return current + incoming.slice(overlap);
+  }
+
+  if (shouldReplaceWithSnapshot(current, incoming)) {
+    return incoming;
+  }
+
+  return current + incoming;
+}