fix: repair streaming output broken by markdown rendering changes

Two bugs were introduced when sendTextMessage was changed to auto-detect
message format (text/post/interactive):

1. Message type mismatch: sendTextMessage could create a text or post
   message, but patchTextMessage only works on interactive cards.
   Fix: add sendInteractiveCardMessage for streaming create (always
   interactive, always patchable).

2. flushTextToSink state corruption: the text management after flush
   had a flawed appendedDuringFlush heuristic that could lose text or
   duplicate it. Fix: properly track sent vs unsent text by removing
   the sent prefix length, keeping only the last chunk for future patches.

Also fix finish() to flush pending text regardless of currentMessageId.

hub/src/feishu/client.ts

@@ -234,6 +234,24 @@ export async function sendText(rt: FeishuRuntime, chatId: string, text: string):
   await sendTextMessage(rt, chatId, text);
 }
 
+/** Send an interactive card message (always patchable). Used for streaming. */
+export async function sendInteractiveCardMessage(rt: FeishuRuntime, chatId: string, text: string): Promise<string | null> {
+  const payload = buildOutboundPayload(text);
+  const card = buildInteractiveCardFromOutboundPayload(payload);
+  const client = rt.client as unknown as {
+    im: { v1: { message: { create: (p: unknown) => Promise<MessageCreateResponse> } } };
+  };
+  try {
+    const res = await client.im.v1.message.create({
+      params: { receive_id_type: "chat_id" },
+      data: { receive_id: chatId, msg_type: "interactive", content: JSON.stringify(card) },
+    });
+    return messageIdFromResponse(res);
+  } catch {
+    return null;
+  }
+}
+
 /** Patch a text-as-card message in-place (streaming updates). */
 export async function patchTextMessage(rt: FeishuRuntime, messageId: string, text: string): Promise<void> {
   const payload = buildOutboundPayload(text);