Add missing docstring to get_cache_path function

[Copilot]

Ruff linter flagged get_cache_path() in src/ffmpeg/common/cache.py for missing docstring (D103 violation).

Changes

• Added Google-style docstring to get_cache_path() documenting its behavior for both frozen (PyInstaller) and normal execution contexts

Original prompt

---

This section details on the original issue you should resolve

<issue_title>linter issues?</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes linter issues? #807

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.98%. Comparing base (03b7c35) to head (071770b).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (79.98%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #809   +/-   ##
=======================================
  Coverage   79.98%   79.98%           
=======================================
  Files          51       51           
  Lines        3432     3432           
=======================================
  Hits         2745     2745           
  Misses        687      687           

Flag	Coverage Δ
3.10	79.98% <ø> (ø)
3.11	79.95% <ø> (ø)
3.12	79.95% <ø> (ø)
backend	79.98% <ø> (ø)
python	79.98% <ø> (ø)

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

Files with missing lines	Coverage Δ
src/ffmpeg/common/cache.py	93.33% <ø> (ø)

🚀 New features to boost your workflow:

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