fix(bridge): session_runner 将 stderr 合并到 stdout 流

将子进程 stderr=PIPE 改为 stderr=STDOUT，确保 stderr
输出不会因无人读取而阻塞子进程，同时简化调用方对输出流的处理

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/openharness/bridge/session_runner.py

@@ -41,6 +41,6 @@ async def spawn_session(
         command,
         cwd=resolved_cwd,
         stdout=asyncio.subprocess.PIPE,
-        stderr=asyncio.subprocess.PIPE,
+        stderr=asyncio.subprocess.STDOUT,
     )
     return SessionHandle(session_id=session_id, process=process, cwd=resolved_cwd)

tests/test_bridge/test_core.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from pathlib import Path
+import asyncio
 
 import pytest
 
@@ -27,3 +28,34 @@ async def test_spawn_session_and_kill(tmp_path: Path):
     assert handle.process.returncode is None
     await handle.kill()
     assert handle.process.returncode is not None
+
+
+@pytest.mark.asyncio
+async def test_spawn_session_merges_stderr_into_stdout(monkeypatch, tmp_path: Path):
+    seen_kwargs = {}
+
+    class FakeProcess:
+        returncode = 0
+
+        def terminate(self):
+            pass
+
+        def kill(self):
+            pass
+
+        async def wait(self):
+            return 0
+
+    async def fake_create_shell_subprocess(*args, **kwargs):
+        seen_kwargs.update(kwargs)
+        return FakeProcess()
+
+    monkeypatch.setattr(
+        "openharness.bridge.session_runner.create_shell_subprocess",
+        fake_create_shell_subprocess,
+    )
+
+    await spawn_session(session_id="s1", command="printf err >&2", cwd=tmp_path)
+
+    assert seen_kwargs["stdout"] is asyncio.subprocess.PIPE
+    assert seen_kwargs["stderr"] is asyncio.subprocess.STDOUT