fix: flush block reply coalescer on tool boundaries

When block streaming is enabled with verbose=off, tool blocks are hidden
but their boundary information was lost. Text segments before and after
tool execution would get coalesced into a single message because the
coalescer had no signal that a tool had executed between them.

This adds an onBlockReplyFlush callback that fires on tool_execution_start,
allowing the block reply pipeline to flush pending text before the tool
runs. This preserves natural message boundaries even when tools are hidden.

Fixes the issue where:
  text → [hidden tool] → text → rendered as one merged message

Now correctly renders as:
  text → [hidden tool] → text → two separate messages

Co-diagnosed-by: Krill (Discord assistant)

src/agents/pi-embedded-subscribe.ts

@@ -198,6 +198,8 @@ export function subscribeEmbeddedPiSession(params: {
     mediaUrls?: string[];
     audioAsVoice?: boolean;
   }) => void | Promise<void>;
+  /** Flush pending block replies (e.g., before tool execution to preserve message boundaries). */
+  onBlockReplyFlush?: () => void | Promise<void>;
   blockReplyBreak?: "text_end" | "message_end";
   blockReplyChunking?: BlockReplyChunking;
   onPartialReply?: (payload: {
@@ -483,6 +485,11 @@ export function subscribeEmbeddedPiSession(params: {
       }
 
       if (evt.type === "tool_execution_start") {
+        // Flush pending block replies to preserve message boundaries before tool execution
+        if (params.onBlockReplyFlush) {
+          void params.onBlockReplyFlush();
+        }
+
         const toolName = String(
           (evt as AgentEvent & { toolName: string }).toolName,
         );