fix(agents): strip [Historical context: ...] and tool call text from streaming path (#13453)

- Add [Historical context: ...] marker pattern to stripDowngradedToolCallText
- Apply stripDowngradedToolCallText in emitBlockChunk streaming path
- Previously only stripBlockTags ran during streaming, leaking [Tool Call: ...] markers to users
- Add 7 test cases for the new pattern stripping

src/agents/pi-embedded-subscribe.ts

@@ -15,7 +15,7 @@ import {
   normalizeTextForComparison,
 } from "./pi-embedded-helpers.js";
 import { createEmbeddedPiSessionEventHandler } from "./pi-embedded-subscribe.handlers.js";
-import { formatReasoningMessage } from "./pi-embedded-utils.js";
+import { formatReasoningMessage, stripDowngradedToolCallText } from "./pi-embedded-utils.js";
 import { hasNonzeroUsage, normalizeUsage, type UsageLike } from "./usage.js";
 
 const THINKING_TAG_SCAN_RE = /<\s*(\/?)\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
@@ -449,7 +449,8 @@ export function subscribeEmbeddedPiSession(params: SubscribeEmbeddedPiSessionPar
       return;
     }
     // Strip <think> and <final> blocks across chunk boundaries to avoid leaking reasoning.
-    const chunk = stripBlockTags(text, state.blockState).trimEnd();
+    // Also strip downgraded tool call text ([Tool Call: ...], [Historical context: ...], etc.).
+    const chunk = stripDowngradedToolCallText(stripBlockTags(text, state.blockState)).trimEnd();
     if (!chunk) {
       return;
     }