Address Zizmor findings and enable Zizmor in CI

[tnytown]

Resolves #918.

Describe your changes
This changeset addresses Zizmor findings and adds Zizmor to CI.

Testing performed
I ran Zizmor with uvx zizmor .github/ --gh-token=$(gh auth token) --persona=pedantic --fix=all and fixed relevant issues that were not auto-fixable before opening this PR.

Additional context
We also configure Dependabot here to bump Docker image versions. Let me know if this isn't desired.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.39%. Comparing base (4a99614) to head (30aa904).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #922      +/-   ##
==========================================
- Coverage   92.43%   92.39%   -0.04%     
==========================================
  Files          99       99              
  Lines       11785    11785              
  Branches      429      429              
==========================================
- Hits        10893    10889       -4     
- Misses        892      896       +4     

Flag	Coverage Δ
cpp	92.39% <ø> (-0.04%)	⬇️
python_and_cython	92.39% <ø> (-0.04%)	⬇️

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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[tnytown]

codecov-action seems to support OIDC if you'd like to remove the static secret: https://github.com/codecov/codecov-action#using-oidc

[godlygeek]

LGTM, thanks @tnytown!