Fix ResourceWarning leak for internally opened streams

[ReinerBRO]

Summary

• register a finalizer to close internally-opened PDF streams if a PDF object is garbage collected without an explicit close
• keep externally provided streams untouched
• add a regression test covering the ResourceWarning leak and external-stream behavior

Why

Issue #1336 reports that file handles opened by pdfplumber.open(path) can survive until garbage collection and emit a ResourceWarning. This change ensures internally owned streams are still closed if callers forget to use a context manager or call close().

Tests

• . .venv/bin/activate && python -m pytest tests/test_resource_warning.py -q

Closes #1336

[ReinerBRO]

Closing this duplicate PR in favor of #1363, which carries the same commit and targets the intended branch for review.