fix(rivetkit-napi): release runtime_state ref via TSF instead of leaking it

[AlSh007]

Summary

• Fixes (Issue)- unbounded memory growth during actors cycling. #5607: ActorContextShared::reset_runtime_state() (and its Drop impl) run on Tokio worker threads with no N-API Env available, so releasing the runtime_state napi Ref there previously fell back to std::mem::forget. Since this runs on every actor wake cycle, a long-running process with many actors cycling through sleep/wake accumulates leaked V8 reference slots without bound.
• Replaces the unconditional mem::forget with a lazily-installed ThreadsafeFunction that hops onto the JS thread to call Ref::unref properly. The TSF is installed the first time runtime_state(env) is called with a real Env, and is itself unref'd so it doesn't keep the Node process alive. The mem::forget fallback is kept only for the case where the dropper was never installed (e.g. a test context with no JS runtime) or where unref itself fails (e.g. environment teardown), avoiding a debug-mode zero-count assertion panic in napi-rs.
• This matches the fix design already sketched in this repo's own docs-internal/engine/napi-bridge.md notes.

Verification

• cargo check -p rivetkit-napi and cargo clippy -p rivetkit-napi --no-deps are clean.
• Full existing cargo test -p rivetkit-napi --lib suite passes (20/20), including the test that exercises reset_runtime_state.
• Built the real native .node addon (napi build --platform) and, with temporary instrumentation (not included in this diff), confirmed against a real Node process that 50 consecutive runtime_state(env) + reset_runtime_state() cycles resulted in all 50 references being released through the new dropper rather than leaked.
• I did not add a permanent automated integration test for this, since doing so would need either new test-only #[napi] surface or a flake-prone memory-growth assertion. Happy to add one if maintainers have a preferred pattern for this (the napi-bridge.md note mentions wanting a NAPI integration test that "waits for the cleanup TSF to drain, and verifies native reference counts return to zero").

Test plan

• cargo check -p rivetkit-napi
• cargo clippy -p rivetkit-napi --no-deps
• cargo test -p rivetkit-napi --lib
• Manual verification against a real built .node addon (see above)