fix(kiro): validate terminal streams before emitting output

Validate AWS EventStream framing, header bounds, CRCs, error frames,
and terminal stop metadata before exposing Kiro output. Classify stop
reasons into dispositions (complete / retryable / terminal_incomplete /
refusal) and retry once when the stream ends with a malformed tool call,
ellipsis-only output, or a short future-action sentence.

Fail closed: propagate streaming failures as error SSE (502) instead of
collapsing them into a successful stop, so incomplete responses no longer
leak as final answers.

Detect the observed evidence-prefixed trailing progress final without
broadening the Chinese heuristic to completed findings.

tests/unit/kiro-thinking-strip.test.js

@@ -32,10 +32,23 @@ function createMockFrame(eventType, payloadObj) {
   offset += headerValueBytes.length;
 
   buffer.set(payloadBytes, offset);
-  
+
+  view.setUint32(8, crc32(buffer.subarray(0, 8)), false);
+  view.setUint32(totalLength - 4, crc32(buffer.subarray(0, totalLength - 4)), false);
   return buffer;
 }
 
+function crc32(bytes) {
+  let crc = 0xffffffff;
+  for (const byte of bytes) {
+    crc ^= byte;
+    for (let bit = 0; bit < 8; bit++) {
+      crc = (crc >>> 1) ^ ((crc & 1) ? 0xedb88320 : 0);
+    }
+  }
+  return (crc ^ 0xffffffff) >>> 0;
+}
+
 async function readAllSSE(stream) {
   const reader = stream.getReader();
   const decoder = new TextDecoder();
@@ -130,14 +143,16 @@ describe("KiroExecutor thinking tag stripping", () => {
     expect(contentChunks.length).toBe(0);
   });
 
-  it("emits a terminal chunk at messageStop before the upstream stream closes", async () => {
+  it("waits for clean EOF before emitting stop after messageStop", async () => {
     const executor = new KiroExecutor();
 
     const f1 = createMockFrame("assistantResponseEvent", { content: "OK" });
     const f2 = createMockFrame("messageStopEvent", {});
 
+    let upstreamController;
     const readableStream = new ReadableStream({
       start(controller) {
+        upstreamController = controller;
         controller.enqueue(f1);
         controller.enqueue(f2);
       }
@@ -147,11 +162,16 @@ describe("KiroExecutor thinking tag stripping", () => {
     const reader = transformedResponse.body.getReader();
     const decoder = new TextDecoder();
     let output = "";
-    for (let i = 0; i < 4 && !output.includes("\"finish_reason\":\"stop\""); i++) {
-      const { value } = await readNextWithTimeout(reader);
-      output += decoder.decode(value, { stream: true });
+    const { value } = await readNextWithTimeout(reader);
+    output += decoder.decode(value, { stream: true });
+    expect(output).not.toContain("\"finish_reason\":\"stop\"");
+
+    upstreamController.close();
+    while (!output.includes("\"finish_reason\":\"stop\"")) {
+      const { value: nextValue, done } = await readNextWithTimeout(reader);
+      if (done) break;
+      output += decoder.decode(nextValue, { stream: true });
     }
-    await reader.cancel();
 
     expect(output).toContain("\"finish_reason\":\"stop\"");
   });