fix: resolve nightly mypy errors

[sdimitro]

Summary

• Update SDB ELF note extraction typing for pyelftools 0.33 so strict mypy accepts PT_NOTE iteration and custom SDB note types.
• Convert the drgn slab helper result to int to satisfy strict return typing under current mypy.

Test plan

• PYTHONPATH=.tmp-mypy-deps python3 -m mypy --strict --show-error-codes -p sdb
• PYTHONPATH=.tmp-check-deps python3 -m yapf --diff --style google --recursive sdb
• PYTHONPATH=.tmp-check-deps python3 -m ruff check sdb tests

Notes: python3 -m pytest -v tests/unit/test_session.py could not run locally because this macOS Python environment is missing drgn.

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.65%. Comparing base (348f7d6) to head (8ca0678).

Files with missing lines	Patch %	Lines
sdb/session.py	57.14%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #37      +/-   ##
===========================================
+ Coverage    76.64%   76.65%   +0.01%     
===========================================
  Files           73       73              
  Lines         4556     4558       +2     
  Branches       728      728              
===========================================
+ Hits          3492     3494       +2     
  Misses         878      878              
  Partials       186      186              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.