test(rivetkit): update stale native integration fixture

[eersnington]

Description

The native integration test (tests/napi-runtime-integration.test.ts) has been failing on main. The code it tests is fine; the test's fixture app went stale.

The fixture actor requires a userId when you connect, and the test never sent one, so the very first action was rejected. A few smaller things drifted too: the old db.execute argument shape, an extra decode on kv values that are already strings, no clean shutdown on SIGINT, and the untyped error group is rivetkit now instead of core.

This updates the test and fixture to today's API. Test-only, two files, no runtime code.

Related: #5643. The test needs that fix as well to pass; without it, it fails earlier at the Engine health check.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Before (with #5643, old fixture): the Engine starts, the first action fails.

$ pnpm exec vitest run tests/napi-runtime-integration.test.ts
code=validation_error message="An internal error occurred"
 × runs a TS actor through registry, NAPI, core, envoy, and engine

After (with #5643 and this change):

$ pnpm exec vitest run tests/napi-runtime-integration.test.ts
 ✓ runs a TS actor through registry, NAPI, core, envoy, and engine
 Test Files  1 passed (1)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[eersnington]

closing since it's fixed and merged 89c31e9 / #5648