Add basic addEventListener conformance tests

[julianna-ciq]

Describe your change

Adds basic conformance coverage for DesktopAgent.addEventListener:

• BasicAEL1 verifies the method exists, is callable for userChannelChanged, and returns a Listener with an unsubscribe function.
• BasicAEL2 verifies the unfiltered null event type also returns a valid Listener.
• Registers both tests in the basic suite and adds the API documentation link.
• Documents both test definitions in Basic-Tests.md and adds the required changelog entry.

This builds on the approach in #1817 and incorporates the outstanding review feedback there: current main, synchronized conformance documentation, a changelog entry, and explicit test descriptions.

Related Issue

Resolves #1774

Contributor License Agreement

• I acknowledge that a contributor license agreement is required and that I have one in place or will seek to put one in place ASAP.

Review Checklist

• Issue: If a change was made to the FDC3 Standard, was an issue linked above?
• CHANGELOG: Is a CHANGELOG.md entry included?
• API changes: No API changes; this PR adds conformance coverage and synchronized documentation only.

Verification

• npm run build --workspace toolbox/fdc3-conformance
• Prettier check for all changed files
• git diff --check

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (9ff3585) to head (0baf283).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2154   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files          73       73           
  Lines        4251     4251           
  Branches      671      593   -78     
=======================================
  Hits         4040     4040           
  Misses        211      211           

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LGTM, thanks for this 😄