Fix ResourceWarning from unclosed internal file handles (#1336)

[ReinerBRO]

Summary: Fixes #1336 by ensuring internally-opened file streams are closed even when callers do not use a context manager or call close explicitly. This version uses weakref.finalize as a GC safety net and keeps external stream behavior unchanged.

Changes:

• Add _close_stream_if_internal helper in pdfplumber/pdf.py.
• Register a finalizer in PDF.init to close internal streams on GC.
• Route PDF.close through the same finalizer for idempotent cleanup.
• Add tests/test_resource_warning.py regression tests.
• Update CHANGELOG under Unreleased.
• Add contributor entry in README.

Validation:

• .venv/bin/python -m pytest tests/test_resource_warning.py -q
• .venv/bin/python -W error::ResourceWarning -m pytest tests/test_resource_warning.py tests/test_mcids.py -q
• black/isort/flake8 checks on changed Python files.