fix(test): correct assertions in zipkin browser export tests

[overbalance]

Which problem is this PR solving?

Several zipkin browser tests had assertions that silently passed without verifying anything. The tests appeared green but were not actually validating the behavior they claimed to test.

Short description of the changes

• assert.ok(request.url, endpointUrl) was a no-op. assert.ok treats its second argument as a failure message, not a comparison value. The URL was never actually checked. Fixed to assert.strictEqual.

• Environment variable test could never work in browser. The test set window.OTEL_EXPORTER_ZIPKIN_ENDPOINT expecting the exporter to read it, but window.OTEL_* support was removed in the 2.x release (Feb 2025). The browser getStringFromEnv() now unconditionally returns undefined, so the exporter silently fell through to the default URL (localhost:9411). The broken assert.ok masked this. Changed to use config.url, which is the only way to set a custom URL in the browser since 2.x.

• globalErrorHandler tests asserted against the wrong code path. HTTP 400 responses trigger onreadystatechange (not xhr.onerror), so globalErrorHandler was never called. The old tests had no done callback, so the async assertions inside the export callback never ran. Rewritten to assert ExportResultCode.FAILED via the export callback with proper done handling.

• Added two new tests for previously uncovered paths: sendBeacon returning false (browser refused to queue) and xhr.onerror network errors (DNS failure, CORS, connection refused) that call globalErrorHandler.

Type of change

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

How Has This Been Tested?

• Browser tests pass in CI (karma/headless Chrome)
• No regressions in existing zipkin exporter tests

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 95.75%. Comparing base (1cf5ef3) to head (3dc73c4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6566      +/-   ##
==========================================
+ Coverage   95.74%   95.75%   +0.01%     
==========================================
  Files         371      371              
  Lines       12514    12514              
  Branches     2962     2962              
==========================================
+ Hits        11981    11983       +2     
+ Misses        533      531       -2     

see 1 file with indirect coverage changes

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