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-utils.ts

@@ -37,7 +37,7 @@ export function stripDowngradedToolCallText(text: string): string {
   if (!text) {
     return text;
   }
-  if (!/\[Tool (?:Call|Result)/i.test(text)) {
+  if (!/\[Tool (?:Call|Result)/i.test(text) && !/\[Historical context/i.test(text)) {
     return text;
   }
 
@@ -186,6 +186,9 @@ export function stripDowngradedToolCallText(text: string): string {
   // Remove [Tool Result for ID ...] blocks and their content.
   cleaned = cleaned.replace(/\[Tool Result for ID[^\]]*\]\n?[\s\S]*?(?=\n*\[Tool |\n*$)/gi, "");
 
+  // Remove [Historical context: ...] markers (self-contained within brackets).
+  cleaned = cleaned.replace(/\[Historical context:[^\]]*\]\n?/gi, "");
+
   return cleaned.trim();
 }