refactor(js): align tool-sieve segment start and tail window with go

internal/js/helpers/stream-tool-sieve/sieve.js

@@ -197,11 +197,7 @@ function findToolSegmentStart(state, s) {
     }
     const keyIdx = bestKeyIdx;
     const start = s.slice(0, keyIdx).lastIndexOf('{');
-    if (start < 0) {
-      offset = keyIdx + matchedKeyword.length;
-      continue;
-    }
-    let candidateStart = start;
+    let candidateStart = start >= 0 ? start : keyIdx;
     // If the keyword matched inside an XML tag (e.g. "tool_calls" in "<tool_calls>"),
     // back up past the '<' to capture the full tag.
     if (candidateStart > 0 && s[candidateStart - 1] === '<') {

internal/js/helpers/stream-tool-sieve/state.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const TOOL_SIEVE_CONTEXT_TAIL_LIMIT = 4096;
+const TOOL_SIEVE_CONTEXT_TAIL_LIMIT = 256;
 
 function createToolSieveState() {
   return {

tests/node/stream-tool-sieve.test.js

@@ -252,7 +252,7 @@ test('sieve keeps plain text intact in tool mode when no tool call appears', ()
   assert.equal(leakedText, '你好，这是普通文本回复。请继续。');
 });
 
-test('sieve does not start capture on plain "tool_calls" prose without opening json brace', () => {
+test('sieve keeps plain "tool_calls" prose as text when no valid payload follows', () => {
   const events = runSieve(
     ['前置。', '这里提到 tool_calls 只是解释，不是调用。', '后置。'],
     ['read_file'],