Ensure current user channel is resolved before DesktopAgentProxy is used

[kriswest]

Describe your change

Resolves a regression for web-based Desktop Agents that implement their own user channel management (where an app may start already joined to a channel) by:

• Retrieving the current channel from the Desktop Agent on startup. In prior releases this happened on calling addContextListener, but was removed since.
• Preventing a race condition with addContextListener by making DefaultChannelSupport implement Connectable — moving the getUserChannel() and addEventListener initialization calls from fire-and-forget promises in the constructor into a connect() method that is properly awaited via the DesktopAgentProxy's existing connectables lifecycle.
• Reducing redundant requests for the current channel and user channel list by ensuring the caches for each are always used when populated.
• Adding tests for ChannelSupport cases where the Desktop Agent has preset a user channel

Related Issue

resolves #1857

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?

[netlify]

✅ Deploy Preview for fdc3 ready!

Name	Link
🔨 Latest commit	4753fa2
🔍 Latest deploy log	https://app.netlify.com/projects/fdc3/deploys/69e1526742ce1400083c5ae1
😎 Deploy Preview	https://deploy-preview-1858.preview-fdc3.finos.org
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.61%. Comparing base (b753eee) to head (4753fa2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...-agent-proxy/src/channels/DefaultChannelSupport.ts	96.55%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1858    +/-   ##
========================================
  Coverage   95.61%   95.61%            
========================================
  Files          69       69            
  Lines        4675     4680     +5     
  Branches      833      582   -251     
========================================
+ Hits         4470     4475     +5     
  Misses        205      205            

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[kriswest]

@Roaders @julianna-ciq I think this resolves the regression we discussed and fixes an existing race condition. I'll push a v2.2.2-beta.2 with this in to facilitate easier testing.

[Roaders]

LGTM