test(sdk-trace-base): remove obsolete TypeScript and platform workarounds

[overbalance]

Which problem is this PR solving?

Test files contain outdated workarounds for TypeScript < 4.6 and unnecessary platform detection logic.

Short description of the changes

• Remove WebVisibilityState type alias - TypeScript 4.6+ provides DocumentVisibilityState natively
• Replace describeDocument conditional with describeBrowser used elsewhere
• Simplify Node test env setup by removing global.process?.versions?.node check - unnecessary in test/node/

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Existing unit tests pass

Checklist:

• Followed the style guidelines of this project

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.58%. Comparing base (e5f2d42) to head (2a4e63a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6327   +/-   ##
=======================================
  Coverage   95.58%   95.58%           
=======================================
  Files         314      314           
  Lines        9590     9590           
  Branches     2221     2221           
=======================================
  Hits         9167     9167           
  Misses        423      423           

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