Re-glue spaced -D tokens so knob defines are detected like PlatformIO

tests/unit_tests/build_gen/test_arduino8266.py

@@ -665,3 +665,11 @@ def test_write_project_empty_core_raises(tmp_path: Path) -> None:
         pytest.raises(EsphomeError, match="no core sources"),
     ):
         arduino8266.write_project(paths)
+
+
+def test_flag_defines_joins_spaced_define() -> None:
+    """A spaced "-D KNOB" entry is detected exactly as PlatformIO detects it."""
+    _set_flags("-D PIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
+    defines = _flag_defines()
+    assert "PIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH" in defines
+    assert "" not in defines

tests/unit_tests/test_platformio_library.py

@@ -538,5 +538,9 @@ def test_split_flag_entry_unbalanced_quote_is_clean() -> None:
     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")