Refactor ComposeUI Desktop Agent project to be independent of the IMessagingService implementation

[phn-ms]

Adopt C# 12 collection expressions for array and list initialization, update async test methods to return Task, add braces to single-line if statements, and apply minor style improvements (e.g., var usage, u8 string literals, concise record/class definitions). No changes to business logic or test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 66.04938% with 110 lines in your changes missing coverage. Please review.

Project coverage is 65.8%. Comparing base (cc6e766) to head (af99700).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...Stanley.ComposeUI.DesktopAgent/Fdc3DesktopAgent.cs	66.6%	19 Missing and 11 partials ⚠️
...gAdapter.Abstractions/MessagingAdapterException.cs	0.0%	20 Missing ⚠️
...ingAdapter.MessageRouter/MessageRouterMessaging.cs	83.8%	9 Missing and 1 partial ⚠️
...ng/dotnet/src/Client/Client/MessageRouterClient.cs	68.1%	6 Missing and 1 partial ⚠️
...ssaging/dotnet/src/Core/MessageRouterExtensions.cs	36.3%	7 Missing ⚠️
...cture/Internal/Fdc3DesktopAgentMessagingService.cs	85.7%	4 Missing and 1 partial ⚠️
.../Infrastructure/Internal/ResolverUICommunicator.cs	63.6%	4 Missing ⚠️
...nt/Infrastructure/Internal/UserChannelSetReader.cs	20.0%	4 Missing ⚠️
...ions/MessagingAdapterDuplicateEndpointException.cs	0.0%	4 Missing ⚠️
...anStanley.ComposeUI.Utilities/CommandLineParser.cs	20.0%	2 Missing and 2 partials ⚠️
... and 8 more

❌ Your patch status has failed because the patch coverage (66.0%) is below the target coverage (100.0%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1041     +/-   ##
=======================================
- Coverage   65.9%   65.8%   -0.1%     
=======================================
  Files        333     343     +10     
  Lines       9956   10061    +105     
  Branches    1201    1158     -43     
=======================================
+ Hits        6567    6627     +60     
- Misses      3128    3169     +41     
- Partials     261     265      +4     

Files with missing lines	Coverage Δ
...nley.ComposeUI.DesktopAgent/Channels/AppChannel.cs	50.0% <ø> (ø)
...ley.ComposeUI.DesktopAgent/Channels/UserChannel.cs	100.0% <ø> (ø)
...encyInjection/Fdc3DesktopAgentBuilderExtensions.cs	60.0% <100.0%> (ø)
...anley.ComposeUI.DesktopAgent/Fdc3ShutdownAction.cs	68.0% <ø> (ø)
...anley.ComposeUI.DesktopAgent/Protocol/AppIntent.cs	50.0% <ø> (ø)
src/messaging/dotnet/src/Core/MessageBuffer.cs	75.5% <ø> (ø)
...et/src/Core/Protocol/Json/JsonMessageSerializer.cs	75.6% <ø> (ø)
...oseUI.MessagingAdapter.Abstractions/JsonFactory.cs	100.0% <100.0%> (ø)
...agingAdapter.Abstractions/MessageAdapterContext.cs	100.0% <100.0%> (ø)
...dapter.Abstractions/MessageBufferJsonExtensions.cs	100.0% <100.0%> (ø)
... and 22 more

... and 2 files with indirect coverage changes

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[lilla28]

Tests are missing. but I am okay to go forward without that.