sea: handle NUL bytes in asset keys

[Archkon]

Construct the asset lookup string_view with the explicit Utf8Value length so embedded NUL bytes do not truncate keys.

[nodejs-github-bot]

Review requested:

• @nodejs/single-executable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (54a5095) to head (f73674e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64773      +/-   ##
==========================================
+ Coverage   90.14%   90.15%   +0.01%     
==========================================
  Files         744      744              
  Lines      242518   242518              
  Branches    45685    45683       -2     
==========================================
+ Hits       218611   218649      +38     
+ Misses      15396    15364      -32     
+ Partials     8511     8505       -6     

Files with missing lines	Coverage Δ
src/node_sea.cc	89.53% <100.00%> (ø)

... and 24 files with indirect coverage changes

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