Fix TSP None type encoding

[WilliamK112]

Summary

Fixes #4035.

This changes Pyrefly's TSP conversion for None from an off-spec BuiltInType named none to a ClassType instance declared as types.NoneType. The protocol's BuiltInType.name contract only allows sentinel names such as unknown, any, ellipsis, never, and noreturn, so encoding None as a class keeps str | None and other unions reconstructable by TSP consumers.

The change also updates the TSP interaction coverage for x = None so the actual wire response is checked as a class declaration rather than a builtin sentinel.

Test Plan

• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly tsp::type_conversion::tests:: -- --nocapture
• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly test_get_computed_type_none_is_class -- --nocapture
• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly tsp::type_conversion::tests::test_function_declaration_resolves_special_function_via_export -- --exact --nocapture
• RUSTUP_TOOLCHAIN=stable-x86_64-pc-windows-gnu python test.py --mode cargo --no-test --no-tensor-shapes --no-conformance --no-jsonschema
• git diff --check