Address ChannelRuntime callback review comments

[eanzhao]

Summary

• Revert the ADR-0011 edit from PR Remove retired ChannelRuntime direct callback endpoint #673 so the already-superseded ADR remains immutable.
• Dispose the WebApplication instances in the ChannelCallback route mapping tests with await using.

Follow-up for review comments on #673.

Validation

• dotnet test test/Aevatar.GAgents.ChannelRuntime.Tests/Aevatar.GAgents.ChannelRuntime.Tests.csproj --nologo
• bash tools/ci/test_stability_guards.sh
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.41%. Comparing base (c1f4f02) to head (a2a7748).

@@           Coverage Diff           @@
##              dev     #679   +/-   ##
=======================================
  Coverage   82.40%   82.41%           
=======================================
  Files         934      934           
  Lines       59698    59698           
  Branches     7827     7827           
=======================================
+ Hits        49193    49198    +5     
+ Misses       7133     7132    -1     
+ Partials     3372     3368    -4     

Flag	Coverage Δ
ci	82.41% <ø> (+<0.01%)	⬆️

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see 3 files with indirect coverage changes

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