Fix build system to relink when source files change

- Make copy_file_if_changed() return bool indicating if file was copied
- Track sources_changed in copy_src_tree() to detect when source files change
- Only update build_info timestamp when sources/config/version change
- Exclude generated files (build_info_data.h) from sources_changed tracking
- Add build_info_data.h to ignore_targets to prevent copying from resources
- Track changes to generated headers (defines.h, esphome.h, version.h)
- Check for config_hash or version changes to trigger rebuild
- Pretty-print build_info.json with indentation and trailing newline
- Update mock_copy_file_if_changed to return True by default

This fixes the issue where changing a source file would recompile the .o
file but not relink the final program executable.

tests/unit_tests/conftest.py

@@ -58,6 +58,7 @@ def mock_write_file_if_changed() -> Generator[Mock, None, None]:
 def mock_copy_file_if_changed() -> Generator[Mock, None, None]:
     """Mock copy_file_if_changed for core.config."""
     with patch("esphome.core.config.copy_file_if_changed") as mock:
+        mock.return_value = True
         yield mock