util: fix numericSeparator handling for scientific notation

[Fifixex]

This commit fixes incorrect behavior in util.inspect when numericSeparator is enabled and the input number is represented in scientific notation (e.g. 1e-7 or 1e+21).

Previously, scientific notation strings were incorrectly processed by the numeric separator logic, which assumes the presence of a decimal point. This led to malformed output such as:

'1e-.1e-_7'

The issue occurs because the formatting logic attempts to split the numeric string at a decimal point (.), which does not exist in scientific notation. As a result, incorrect string slicing is applied. This change adds a guard for scientific notation strings, ensuring they are returned without applying numeric separator formatting. It also fixes the case where no decimal point exists by treating the value as a plain number string.

Closes: #62981

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (4e332e0) to head (019ec6e).
⚠️ Report is 370 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62983      +/-   ##
==========================================
+ Coverage   89.65%   89.68%   +0.02%     
==========================================
  Files         707      712       +5     
  Lines      219501   221251    +1750     
  Branches    42073    42389     +316     
==========================================
+ Hits       196803   198423    +1620     
- Misses      14587    14653      +66     
- Partials     8111     8175      +64     

Files with missing lines	Coverage Δ
lib/internal/util/inspect.js	99.47% <100.00%> (-0.07%)	⬇️

... and 108 files with indirect coverage changes

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[BridgeAR]

LGTM with my comment being addressed

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73867/