[o11y] Add undefined to tailStream return types

[fhanau]

Returning undefined indicates that tail events for the given worker event can be ignored, which is used to shut down the tail stream early as an optimization.

[ask-bonk]

async tailStream() { return undefined; } produces Promise<undefined>, which this signature still rejects because undefined is only outside the promise. The runtime explicitly awaits promises and treats a fulfilled undefined as the early-stop signal. Please also apply this same return contract to ExportedHandlerTailStreamHandler in src/workerd/api/global-scope.h and regenerate the snapshots; otherwise ordinary default-module tail workers remain unable to use the optimization.

Suggested change

	tailStream?(event: TailStream.TailEvent<TailStream.Onset>): TailStream.TailEventHandlerType | Promise<TailStream.TailEventHandlerType> | undefined;
	tailStream?(event: TailStream.TailEvent<TailStream.Onset>): TailStream.TailEventHandlerType | Promise<TailStream.TailEventHandlerType | undefined> | undefined;

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

Adds undefined to the RPC tailStream return types.

1. P1: The type still excludes promised undefined, and default-module handlers remain unchanged. Posted 1 inline suggestion.

github run

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉