test: add big-endian round-trip coverage for FIT field patching

tests/fit/test_rewriter_patching.py

@@ -30,6 +30,17 @@ def _build_fixture() -> bytes:
     return make_fit(records)
 
 
+def _build_big_endian_fixture() -> bytes:
+    """Big-endian (architecture=1) file_id definition/data pair. Closes the
+    big-endian coverage gap deferred from Task 2: no fixture anywhere in the FIT
+    test suite previously passed endian=">" to definition()/struct.pack, so the
+    architecture-byte branch in _read_definition (and the endian format string it
+    threads into every field read/write) was never actually exercised."""
+    file_def = definition(0, FILE_ID_MESG_NUM, [(1, 2, 0x84), (2, 2, 0x84)], endian=">")
+    file_data = data(0, struct.pack(">HH", 255, 999))
+    return make_fit(file_def + file_data)
+
+
 def _build_no_device_index_fixture() -> bytes:
     """device_info record entirely missing field 0 (device_index) must be left untouched."""
     file_def = definition(0, FILE_ID_MESG_NUM, [(1, 2, 0x84), (2, 2, 0x84)])
@@ -161,6 +172,24 @@ def test_convert_fit_device_patches_product_name_string_fields(tmp_path: Path) -
     assert values_by_key[(DEVICE_INFO_MESG_NUM, 27)] == "Edge 1030 Plus"
 
 
+def test_convert_fit_device_round_trips_big_endian_fields(tmp_path: Path) -> None:
+    """Proves both the read side (parsing manufacturer/product under architecture=1)
+    and the write side (patching them back in big-endian byte order) are correct --
+    a byte-order bug on either side would flip 255/999 or the patched 1/3570 into an
+    unrelated value once read back with the (still big-endian) definition."""
+    source = tmp_path / "source.fit"
+    source.write_bytes(_build_big_endian_fixture())
+    output = tmp_path / "output.fit"
+
+    convert_fit_device(source, output)
+
+    values = read_device_field_values(output)
+    values_by_key = {(v.global_message_num, v.field_num): v.value for v in values}
+
+    assert values_by_key[(FILE_ID_MESG_NUM, 1)] == 1
+    assert values_by_key[(FILE_ID_MESG_NUM, 2)] == 3570
+
+
 def test_convert_fit_device_defaults_to_edge_1030_plus() -> None:
     device = GarminDevice()
     assert device.manufacturer_id == 1