Clean up .h5 test artifact from test_warn_for_nwb_extension

[rly]

Summary

• An earlier commit accidentally included a test file artifact that was not properly cleaned up.
• test_warn_for_nwb_extension in TestNWBHDF5IO creates a .h5 file but tearDown only cleaned up the .nwb file, leaving test_pynwb_io_nwbhdf5.h5 behind
• Add .h5 cleanup to tearDown, remove the accidentally committed file, and add *.h5 to .gitignore

Test plan

• Verify test_warn_for_nwb_extension still passes
• Verify no .h5 file is left behind after test run

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (d0393e9) to head (4726254).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2169   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files          29       29           
  Lines        2945     2945           
  Branches      757      757           
=======================================
  Hits         2804     2804           
  Misses         84       84           
  Partials       57       57           

Flag	Coverage Δ
integration	72.93% <ø> (ø)
unit	85.16% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 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.

[bendichter]

@rly how big was the file? Big enough that we should consider a rebase?

[bendichter]

Oh, I see, 184KB. OK, not worth it.