Fix dependency property generator default value emission

[Sergio0694]

Summary

Fixes two dependency property generator emitter defects:

• Place cached property initializers after the property's closing brace, rather than inside the accessor block (CS1014).
• Use a static lambda to adapt default factories whose return values require boxing (CS0407), while preserving direct method groups for object and reference returns.
• Include the boxing requirement in the incremental model so factory return-type changes invalidate generated output.

Regression coverage

• Compile and emit generated assemblies for cached int, int?, bool, and string properties, with and without initializers.
• Cover value, nullable, object, reference, and constrained/unconstrained generic factory returns across all four property-changed callback combinations and generic/non-generic containing classes.
• Cover incremental invalidation when a factory changes between int and object returns.
• Correct the existing snapshots and add a dedicated emit assertion, preserving snapshot-only support for existing invalid-input fixtures.

Scope

Organized into two commits: emitter fixes with corrected snapshots, followed by compilation and incrementality regression coverage. No SDK, package, feed, project-file, or CsWinRT multi-targeting changes.