Cover the testing-mode flash-ld OSError guard

tests/unit_tests/build_gen/test_arduino8266.py

@@ -346,6 +346,20 @@ def test_generate_ld_scripts_testing_mode(tmp_path: Path) -> None:
     assert "len = 0x2000000" in patched
 
 
+def test_generate_ld_scripts_testing_mode_missing_flash_ld_raises(
+    tmp_path: Path,
+) -> None:
+    """A missing flash ld in testing mode names the file and the fix."""
+    paths = _make_framework(tmp_path)
+    CORE.testing_mode = True
+    result = MagicMock(returncode=0, stdout=_COMMON_LD_H_OUTPUT, stderr="")
+    with (
+        patch.object(arduino8266.subprocess, "run", return_value=result),
+        pytest.raises(EsphomeError, match="Could not read .*clean-all"),
+    ):
+        _run_generate_ld_scripts(paths)
+
+
 def test_build_config_nonosdk_precedence() -> None:
     """With two SDK knobs set (a pathological config), ties break
     deterministically by table order."""