Enable RefOption returns from Views to support Views returning a single class

[rekhoff]

Description of Changes

This PR adds a regression test covering nullable reference-type view returns in C# modules (e.g. Account? where Account is a class), as reported in #3962.

public static Account? MyAccount(ViewContext ctx)

• Updated the C# regression-test server module to include a reference-type table and views that exercise the RefOption path:
• A new public reference-type table row:

[SpacetimeDB.Table(Name = "account", Public = true)]
public partial class Account { ... }

• A public at-most-one view that returns a nullable reference type (Account?) via Find(...):

[SpacetimeDB.View(Name = "my_account", Public = true)]
public static Account? MyAccount(ViewContext ctx)
{
    return ctx.Db.account.Identity.Find(ctx.Sender) as Account;
}

• A second public view that returns null to ensure the “empty result” case is exercised:

[SpacetimeDB.View(Name = "my_account_missing", Public = true)]
public static Account? MyAccountMissing(ViewContext ctx) => null;

• Updated ClientConnected to ensure an Account row exists for the connecting identity so the “one row” case is deterministic.
• Updated the C# regression-test client to:
  • Subscribe to the new views:
    • SELECT * FROM my_account
    • SELECT * FROM my_account_missing
  • Assert correct semantics for nullable reference-type view returns:
    • MyAccount.Count == 1
    • MyAccountMissing.Count == 0
  • Updated the regression-test server project to use local C# runtime/codegen project references so the regression module exercises the in-repo generator/runtime behavior (instead of the published SpacetimeDB.Runtime package).

API and ABI breaking changes

None.

• No changes to public module schema/wire format semantics beyond adding regression-test-only tables/views.
• No behavior changes outside the C# regression test module + harness.

Expected complexity level and risk

2 - Low

• Changes are isolated to regression tests and project wiring.
• The scenario specifically guards the nullable reference-type “Option-like view return” path against regressions.

Testing

• Ran C# regression tests with no failures in new View tests