[kunalgoesbyken]

Description

Fixes #5891.

• An action that reaches @rivetkit/effect after onSleep or onDestroy has removed the actor instance, but before the request's abort signal fires, now rejects with actor.aborted instead of a plain Error("actor instance missing").
• Core no longer turns that lifecycle exit into rivetkit.internal_error (HTTP 500), so clients that retry on actor.aborted recover across graceful worker restarts.
• The missing-instance branch now matches how the dispatcher already reports interrupted action fibers.
• Adds dispatcher unit tests for a missing instance with a live signal and with an already aborted signal.

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?

• vitest run src/internal/ActionDispatcher.test.ts: the live-signal case fails on main (expected Error: actor instance missing to be an instance of RivetError) and passes with this change.
• vitest run src --typecheck: no type errors. Every suite passes except Registry.test.ts, which needs the native NAPI addon I did not build locally and fails the same way on main.
• test/e2e.test.ts needs a local rivet-engine build and was left to CI.
• tsc --noEmit in rivetkit-typescript/packages/effect passes.
• biome check passes on the changed files.

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 (except the native-only suites noted above)