feat: add fragment variables support

[Jeroen-VdB]

Implements #253 using typed fragment variables, allowing fragments to declare inputs via [FragmentVariable]-decorated properties and callers to pass values through object initializer syntax.

Changes:

• FragmentVariableAttribute (new) — marks a fragment class property as a variable, with optional custom name and default value
• IInboundContext / IOutboundContext / IBackendContext / IOnErrorContext / IFragmentContext — added IncludeFragment(TFragment fragment) where TFragment : IFragment overload
• CompilerUtils — ProcessParameter now resolves [FragmentVariable]-decorated property references inside fragment methods to @(context.Variables.GetValueOrDefault(...)) expressions
• IncludeFragmentCompiler — handles object creation arguments by emitting for each initialized property (respecting [FragmentVariable] variable name) before
• IncludeFragmentHandler (Emulator Testing) — routes IFragment instances directly to ExecuteFragment, bypassing the registry/discovery lookup
• Added tests in IncludeFragmentTests and DocumentTypeTests
• Updated the example to use the new attribute and show how to write it and proves unit testing works