fix(instrumentation-fetch): use feature detection for browser environment

[overbalance]

Which problem is this PR solving?

The current browser detection uses Node.js-specific checks (process.release?.name === 'node'), which doesn't account for other server-side runtimes like Deno and Bun that also have fetch(). This browser-only instrumentation shouldn't attempt to patch fetch in any server-side environment.

Short description of the changes

• Replace Node.js detection with browser feature detection (typeof PerformanceObserver !== 'undefined')
• Use named imports instead of namespace imports for @opentelemetry/api
• Update warning message to be runtime-agnostic ("server-side fetch" instead of "Node.js's fetch")

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Existing browser tests cover this functionality. The feature detection approach checks for PerformanceObserver, which the instrumentation already depends on.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.43%. Comparing base (6321f48) to head (88eb1f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6200   +/-   ##
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  Coverage   95.43%   95.43%           
=======================================
  Files         317      317           
  Lines        9521     9521           
  Branches     2197     2197           
=======================================
  Hits         9086     9086           
  Misses        435      435           

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