fix(dotnet): harden Native AOT serialization

[David Fowler (davidfowl)]

Summary

• preserve canonical AHP camel-case and null-emission wire settings when callers compose custom System.Text.Json metadata resolvers
• serialize erased inbound handler results through runtime source-generated metadata, including reflection-disabled typed, raw, and null responses
• exercise initialize, reconnect replay, subscriptions, reducers, generic traffic, and inbound requests in a deterministic Native AOT executable built from the packed NuGet packages
• make CI pack before Native AOT validation and restore the consumer from an isolated local package cache

Validation

• Release builds for net8.0 and netstandard2.0: 0 warnings, 0 errors
• full .NET suite: 525 passed
• dotnet format whitespace --verify-no-changes --no-restore
• npm run verify:generated
• npm run verify:change-fragments
• npm test
• dotnet pack for both .NET packages, with package layout and assembly metadata inspection
• reflection-disabled win-x64 Native AOT publish and executable run from project references
• reflection-disabled win-x64 Native AOT publish and executable run from a fresh-cache restore of the locally packed packages

Relationship to #414

This follow-up is intentionally complementary to #414. It does not change generators, generated discriminant initializers, or #414's tests/changelog, and it has no merge dependency on #414. If #414 merges first, the branches should remain non-overlapping.