Extract the shared native-build helpers from the ESP-IDF backend

tests/unit_tests/test_platformio_library.py

@@ -531,3 +531,25 @@ def test_convert_libraries_skips_incompatible_dependency(tmp_path, monkeypatch):
     top = convert_libraries([Library("esphome/A", "1.0.0", None)], _backend())
 
     assert top[0].dependencies == []
+
+
+def test_split_flag_entry_unbalanced_quote_is_clean() -> None:
+    """A malformed flags entry raises EsphomeError, not a raw ValueError."""
+    from esphome.platformio.library import split_flag_entry
+
+    assert split_flag_entry('-DX="a b"', "library x") == ["-DX=a b"]
+    # join_flag_args re-glues a spaced -D like ParseFlags does
+    from esphome.platformio.library import join_flag_args
+
+    assert join_flag_args(["-D", "FOO=1", "-Os"], "x") == ["-DFOO=1", "-Os"]
+    with pytest.raises(EsphomeError, match=r"Malformed build flag.*library x"):
+        split_flag_entry('-DX="unclosed', "library x")
+
+
+def test_join_flag_args_trailing_bare_flag_warns(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    from esphome.platformio.library import join_flag_args
+
+    assert join_flag_args(["-Os", "-l"], "library x") == ["-Os"]
+    assert "Ignoring trailing '-l'" in caplog.text