fs: accept all valid utf8 values in fast paths

[hamidrezaghavami]

Fixes: #49888

This PR introduces a lightweight isUtf8Encoding helper to ensure fs.readFileSync, fs.writeFileSync, and encodeRealpathResult use the fast C++ path for all valid variations of the utf8 encoding string (utf8, utf-8, UTF8, UTF-8).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (e6a8d06) to head (fccaa91).
⚠️ Report is 348 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #64341 +/- ##
==========================================
- Coverage 92.01% 90.24% -1.77% 
==========================================
Files 379 741 +362 Lines 166972 241706 +74734 Branches 25554 45562 +20008 ==========================================
+ Hits 153639 218128 +64489 - Misses 13041 15107 +2066 - Partials 292 8471 +8179 

Files with missing lines	Coverage Δ
lib/fs.js	98.36% <100.00%> (+3.76%)	⬆️
lib/internal/util/inspect.js	96.94% <100.00%> (+25.39%)	⬆️

... and 551 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.

[anonrig]

I don't think we should land this. The more checks we have, the more performance impact it would create. Do you have any examples out there that is impacted by this? Does people really pass UTF8? If not, let's close this.

[hamidrezaghavami]

Hi @anonrig thanks for the review!

I completely understand the concern regarding the performance impact on the fast path. I picked this up to resolve issue #49888, as there seemed to be some inconsistency in how variations like utf-8 (which is common in HTTP headers/HTML meta tags) bypass the C++ fast path and fall back to the slower route.

That said, if the consensus is that the branch/check overhead for the 99% of users passing standard utf8 outweighs the benefit for the edge cases, I am perfectly happy to close this PR. Let me know what you think is best for the module!

[hamidrezaghavami]

The commit message linting and sign-off trailer are fixed, and all CI checks are passing! Could someone from the @nodejs/fs team please take a look when you have a moment? Thanks!