ffi: refresh cached string buffers on every call

[trivikr]

Fixes: #65050

Native code can mutate temporary string storage during an FFI call. Rewrite cached buffers on every conversion so a later call with the same JavaScript string receives a fresh copy of its UTF-8 bytes.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (f7804f7) to head (a88fe4c).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65051      +/-   ##
==========================================
- Coverage   90.31%   90.29%   -0.02%     
==========================================
  Files         759      759              
  Lines      247618   247623       +5     
  Branches    46694    46693       -1     
==========================================
- Hits       223632   223588      -44     
- Misses      15454    15503      +49     
  Partials     8532     8532              

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	95.54% <ø> (+1.58%)	⬆️

... and 35 files with indirect coverage changes

🚀 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.