src: reuse cached env strings in remaining files

[leah-1ee]

Follow-up to #59891, covering two more locations it missed:

• src/node_locks.cc: "shared" / "exclusive"
• src/node_constants.cc: "errno"

Two other candidates (node_ffi.cc, crypto_util.cc) were already
covered by #64760, landed just before this branch was pushed.

No behavior change.

Refs: #59891
Refs: #64760
Refs: #64912

[Copilot]

Pull request overview

This PR continues the effort to replace repeated FIXED_ONE_BYTE_STRING(...) literals with Environment/IsolateData-cached strings in the Node.js core C++ sources, improving reuse and consistency without changing behavior.

Changes:

• Reuse IsolateData cached "shared" / "exclusive" strings when defining lock mode constants.
• Reuse the cached "errno" string when defining OS constants.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/node_locks.cc	Uses isolate_data->shared_string() / exclusive_string() instead of new fixed string literals when exporting lock mode constants.
src/node_constants.cc	Uses env->errno_string() instead of creating a fixed string literal for the "errno" key in OS constants.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (41afbd3) to head (ae5c463).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65039   +/-   ##
=======================================
  Coverage   90.30%   90.30%           
=======================================
  Files         759      759           
  Lines      247597   247593    -4     
  Branches    46679    46676    -3     
=======================================
+ Hits       223591   223593    +2     
- Misses      15469    15471    +2     
+ Partials     8537     8529    -8     

Files with missing lines	Coverage Δ
src/node_constants.cc	99.71% <100.00%> (-0.01%)	⬇️
src/node_locks.cc	72.67% <100.00%> (ø)

... and 21 files with indirect coverage changes

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