fix: preserve nullable null branches in generated rewrites

[ycherkes]

Preserve nullable null branches in generated rewrites

Summary

Fix generated CS8600 warnings in the Experiment project without restoring nullable-warning suppression.

The null-conditional Rewrite path previously emitted a null branch cast to a non-nullable reference type when Roslyn reported a non-nullable converted type. That produced generated syntax such as:

(global::Example.AddressDto)null

The rewriter now preserves the nullable result type of the conditional access:

(global::Example.AddressDto?)null

This is type-correct for a null-conditional branch and avoids warnings without adding ! or null!.

Changes

• Use the already computed nullable cast type for every Rewrite null branch.
• Preserve project-level nullable-flow behavior; no warning suppression was reintroduced.
• Update the prior nullable-policy PR summary to reflect the warning-free Experiment build.

Validation

• dotnet build AlephMapper.slnx --configuration Release — 0 warnings, 0 errors
• dotnet test AlephMapper.slnx --configuration Release — 97 passed