fix: guard source resource lookup before init

[fengjikui]

Fixes #4107.

What changed

• Make DltSource.__getattr__ read _resources directly from __dict__ before attempting resource-name lookup.
• Raise AttributeError cleanly when _resources is missing, which can happen for partially initialized / restored objects.
• Add a regression test for a DltSource.__new__(DltSource) instance accessing both a normal missing attribute and _resources itself.

Why

__getattr__ previously accessed self._resources while handling a missing attribute. If _resources was not present yet, that lookup recursively called __getattr__("_resources") until Python raised RecursionError.

The new guard avoids re-entering the attribute machinery and preserves the existing resource lookup behavior once _resources exists.

Validation

• Reproduced on current origin/devel before the fix: DltSource.__new__(DltSource).anything raised RecursionError.
• uv run pytest tests/extract/test_sources.py::test_source_getattr_without_resources_raises_attribute_error tests/extract/test_sources.py::test_select_resources tests/extract/test_sources.py::test_clone_source -q -> 3 passed
• uv run ruff check dlt/extract/source.py tests/extract/test_sources.py -> All checks passed!
• uv run ruff format --check dlt/extract/source.py tests/extract/test_sources.py -> 2 files already formatted
• git diff --check -> clean