fix(test): correct test_sanitize_agent_error_stderr_and_exc assertions

The test was asserting `"ValueError" not in out`, but the implementation
correctly uses exc type as the tag (e.g. "Agent error (ValueError): ...").
The comment even said "exc type is the tag" — the assertion was inverted.

Also added assertion that the exc body ("this should not appear") is NOT
in the output, verifying the body comes from stderr as intended.

workspace/tests/test_executor_helpers.py

@@ -761,8 +761,9 @@ def test_sanitize_agent_error_stderr_and_exc():
     """exception + stderr: exc type is the tag, stderr is the body."""
     err = ValueError("this should not appear")
     out = sanitize_agent_error(exc=err, stderr="rate limit exceeded")
-    assert "ValueError" not in out  # exc class is overridden by stderr
-    assert "rate limit exceeded" in out
+    assert "ValueError" in out  # exc type is the tag
+    assert "rate limit exceeded" in out  # stderr is the body
+    assert "this should not appear" not in out  # exc body is NOT leaked
 
 
 def test_sanitize_agent_error_stderr_empty_string():