fix(engine-runner): terminate requests for missing actors

[Sms1818]

Fixes a request leak in the TypeScript engine runner when a request arrives for an actor that is no longer loaded.

Previously, #handleRequestStart() logged that the request would leak and returned without sending a terminal response. This left the gateway waiting for a response until its response-start timeout.

The runner now sends a retryable 503 Service Unavailable response with the x-rivet-error: runner.actor_not_found header when the actor cannot be found. This follows the existing missing-actor retry semantics used elsewhere in the runner and allows Guard to retry the request instead of waiting for the timeout.

A regression test was added to verify that requests for unloaded actors receive the expected terminal response.

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?

Added a focused regression test covering a ToClientRequestStart received for an unloaded actor.

Verified with:

• pnpm exec vitest run tests/tunnel-missing-actor.test.ts
• pnpm --filter @rivetkit/engine-runner check-types
• pnpm --filter @rivetkit/engine-runner build
• git diff --check

The regression test verifies that the runner returns:

• HTTP status 503
• x-rivet-error: runner.actor_not_found
• the expected response body and content length
• a terminal non-streaming response

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

[Copilot]

Pull request overview

This PR fixes a request leak in the TypeScript engine runner tunnel path by ensuring that a ToClientRequestStart for an actor that is no longer loaded receives a terminal response (instead of returning early and leaving the gateway to time out). This aligns the tunnel behavior with the existing missing-actor retry semantics already present in the runner’s fetch path.

Changes:

• Update Tunnel.#handleRequestStart() to send a retryable 503 response with x-rivet-error: runner.actor_not_found when getAndWaitForActor() returns no actor.
• Add a regression test verifying that missing-actor tunnel requests receive the expected non-streaming terminal response.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rivetkit-typescript/packages/engine-runner/src/tunnel.ts	Sends a terminal 503 + x-rivet-error: runner.actor_not_found response when a request arrives for an unloaded actor.
rivetkit-typescript/packages/engine-runner/tests/tunnel-missing-actor.test.ts	Adds a focused regression test for the missing-actor tunnel request response semantics.

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