Fixed Nullable value-type fields in Views in C# module causing excessive logging

[rekhoff]

Description of Changes

This PR fixes a C# codegen performance/behavior issue triggered by views that include nullable value-type fields (e.g. DbVector2?), as reported in #3914.

• Updated the C# BSATN code generator to emit non-boxing equality for Nullable<T> members by using System.Nullable.Equals(a, b) rather than a.Equals(b) (which can box and cause excessive host logging/work in view diff/equality paths).

This causes code generation to change from something like:

// From:
var ___eqNullableIntField = this.NullableIntField.Equals(that.NullableIntField);
// To:
var ___eqNullableIntField = System.Nullable.Equals(
    this.NullableIntField,
    that.NullableIntField
);

• Added a regression scenario to the C# regression-test module that exercises the problematic pattern:
  • A table containing a nullable struct field (DbVector2? Pos).
  • A public view that returns rows containing that nullable field.
  • A reducer to mutate the nullable field from some → none → some to force view re-evaluation and diffing.
• Updated the regression-test client to:
  • Subscribe to the new view.
  • Validate that the view evaluates successfully and contains the expected rows.
  • Call the reducer and validate view state after updates.
  • Fail the test if view evaluation/diffing produces errors.

API and ABI breaking changes

None.

• No changes to public SpacetimeDB schema or wire format semantics.
• Changes are limited to generated equality code and regression tests.

Expected complexity level and risk

2 - Low

• The codegen change is small and localized (special-casing System.Nullable<T> equality generation).
• Risk is primarily around subtle behavior differences in equality for nullable value types; however, System.Nullable.Equals matches the expected semantics and avoids boxing.
• Regression tests specifically cover the nullable-struct-in-view scenario to guard against regressions.

Testing

• Ran the C# regression test suite via run-regression-tests.sh and confirmed no errors.

[JasonAtClockwork]

Solid improvement with additional testing! I ran the tests as well with no difficulties.