fix(rivetkit): track serverless application listener for graceful shutdown drain

[Tyagiquamar]

Description

When registry.listen() is invoked in serverless listener mode, it starts runtime.serveListener(registry, ...). During graceful shutdown (registry.shutdown() / #drain()), the drain logic specifically includes:

if (this.#applicationListenerPromise !== undefined) {
    await this.#applicationListenerPromise.catch(() => undefined);
}

However, listen() previously called await runtime.serveListener(...) directly without storing the returned promise into this.#applicationListenerPromise. As a result, this.#applicationListenerPromise remained undefined, and shutdown() would not await or drain the active serverless listener before completing.

This PR assigns this.#applicationListenerPromise = listenerPromise; in listen() prior to awaiting it, ensuring the active serverless listener is tracked and drained as intended during shutdown.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added a unit test in rivetkit-typescript/packages/rivetkit/tests/registry-shutdown.test.ts (waits for in-flight serverless application listener before resolving) asserting that shutdown() waits on the active serveListener promise during drain.
• Ran pnpm --filter rivetkit test tests/registry-shutdown.test.ts (all 8 tests pass).
• Formatted with pnpm --filter rivetkit format (Biome).

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• 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