fix: annotate reflection call graph for trimming/NativeAOT compatibility

[akashchamp]

Fixes #539.

Problem

Publishing an app that uses CommandLineApplication.Execute<TApp> with
PublishTrimmed=true (or PublishAot=true) either warns during publish (once
trim analysis is enabled) or throws at run time, e.g.:

Unhandled exception. System.InvalidOperationException: No method named 'OnExecute' or 'OnExecuteAsync' could be found.

Unhandled exception. System.InvalidOperationException: Could not find any public constructors of type 'MyCommand'.

The trimmer removes the constructors/methods/properties that Execute<TApp>,
CommandLineApplication<TModel>, and the attribute conventions discover via
reflection, because none of that reflection was previously annotated with
DynamicallyAccessedMembers.

What this PR does

Following the direction in the maintainer's own WIP commit
(3e0a482),
this adds System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembers
annotations along the reflection call graph reachable from the library's
public entry points, so the trimmer preserves the members those code paths
actually use:

• CommandLineApplication.Execute<TApp>/ExecuteAsync<TApp> (all six
  overloads) and CommandLineApplication<TModel> annotate their model type
  parameter with DynamicallyAccessedMemberTypes.All, and that flows through
  ConventionContext.ModelType, ConstructorInjectionConvention, the
  attribute conventions (SubcommandAttributeConvention,
  OptionAttributeConventionBase, etc.), ValueParserProvider.GetParser,
  CommandOptionTypeMapper, and ReflectionHelper.
• IModelAccessor.GetModelType(), SubcommandMetadata.SubcommandType,
  OptionMetadata.PropertyType/ArgumentMetadata.PropertyType, and
  IMetadataResolver.GetProvider are annotated so metadata already flowing
  through the newer source-generator-aware types (ICommandMetadataProvider,
  DefaultMetadataResolver) carries the same guarantees.
• HashSetParser/ListParser and the validation helpers
  (ValidationExtensions.Satisfies<TAttribute>/GetValidationAttr<T>)
  annotate the Types they construct via Activator.CreateInstance.
• The library opts into trim analysis itself:
  <IsTrimmable>/<EnableTrimAnalyzer> are now set for the net8.0 target
  in McMaster.Extensions.CommandLineUtils.csproj (guarded to that TFM,
  since net472 doesn't support trimming), so a future PR that reintroduces
  an unannotated reflection call will fail the normal build, not just a
  trimmed publish.
• A small number of call sites are irreducibly reflection-based — custom
  TypeConverter-based value parsing (TypeDescriptorValueParserFactory),
  MinLengthAttribute/MaxLengthAttribute's Count-property lookup,
  ValidationContext's constructor, and DefaultMetadataResolver's
  reflection fallback when the source generator hasn't produced metadata for
  a type. These are documented with UnconditionalSuppressMessage and a
  justification, the same pattern already used by DefaultMetadataResolver
  in this codebase and by the BCL's own trimmable libraries.

New sample / regression check

Adds docs/samples/trimming, completing the sample the maintainer started in
the WIP commit above. It publishes self-contained, single-file and trimmed
for linux-x64, with EnableTrimAnalyzer and TreatWarningsAsErrors set, so
a dotnet publish of the sample now fails the build if any reflection in the
call graph it exercises isn't properly annotated — not just a manual demo.
It's wired into docs/samples/samples.sln, which build.ps1 already builds.

Testing

All commands run against this branch on Linux (the CI matrix also covers
Windows/macOS):

• dotnet build CommandLineUtils.sln -c Release — 0 warnings, 0 errors
  (net472 and net8.0, with the net8.0 target's EnableTrimAnalyzer catching
  every reflection call this PR touches).
• dotnet test (both CommandLineUtils.Tests and
  Hosting.CommandLine.Tests, net8.0 and net10.0) — all 1125 + 18 existing
  tests pass unchanged; no tests were removed or weakened.
• pwsh -File build.ps1 -ci (the project's own CI script: format
  verification, full build with -warnaserror:CS1591, pack, the samples
  solution build, and the full test run) — succeeds end to end.
• Manual verification of the actual scenario from the issue:

  dotnet publish docs/samples/trimming/TrimmingSample.csproj -c Release -r linux-x64
  

  publishes cleanly with zero trim warnings (previously this would either
  warn, once analysis was on, or the app would throw at run time), and the
  resulting self-contained trimmed binary runs correctly:

  $ ./docs/samples/trimming/bin/Release/net8.0/linux-x64/publish/TrimmingSample --subject Trimming
  Hello, Trimming!
  $ ./docs/samples/trimming/bin/Release/net8.0/linux-x64/publish/TrimmingSample
  Hello, world!
  $ ./docs/samples/trimming/bin/Release/net8.0/linux-x64/publish/TrimmingSample --help
  Usage: TrimmingSample [options]
  
  Options:
    -s|--subject <SUBJECT>  The subject to greet
                            Default value is: world.
    -?|-h|--help            Show help information.
  

  confirming constructor injection, [Option] binding (including default
  values), OnExecute, and generated help text all survive trimming.

Scope notes

This is scoped to the reflection paths that Execute<TApp> and the attribute
conventions still use today (the same surface the linked WIP commit started
annotating), not a rewrite of the separate source-generator/AOT work already
in src/CommandLineUtils.Generators. docs/samples/aot-sample currently
suppresses trim/AOT warnings wholesale
(SuppressTrimAnalysisWarnings/EnableTrimAnalyzer=false) with a comment
that "the library still uses reflection internally" — this PR doesn't change
that sample, since fully wiring the generator into Execute<TApp>'s default
path is a larger, separate change; happy to look at that as a follow-up if
useful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.13%. Comparing base (00c331e) to head (4b83335).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #616      +/-   ##
==========================================
+ Coverage   80.08%   80.13%   +0.04%     
==========================================
  Files         120      120              
  Lines        4063     4073      +10     
  Branches      880      880              
==========================================
+ Hits         3254     3264      +10     
  Misses        621      621              
  Partials      188      188              

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