presets(deno): add deno-handler preset for library-mode embedding

[petarbasic]

Discussion thread: #4286

Motivation

Multi-tenant Deno hosting (one process serving multiple Nitro builds) needs
a way to embed the Nitro handler in a host process instead of binding a port
per build. The existing deno-server preset calls Deno.serve() as a
module-level side effect — importing the built index.mjs immediately starts
a server, making it unusable as a library.

The Cloudflare preset family already has the equivalent split:
cloudflare-module exports { fetch } instead of side-effecting a listen.
This PR adds the same pattern for the Deno family — a deno-handler sibling
to deno-server.

Use case

A multi-tenant dispatcher routing many Nitro builds in a single Deno
process by Host header, in-process (no localhost TCP):

// dispatcher.ts
import { fetch as siteA } from "./site-a/.output/server/index.mjs";
import { fetch as siteB } from "./site-b/.output/server/index.mjs";

const handlers = new Map([["site-a", siteA], ["site-b", siteB]]);

Deno.serve({ port: 3000 }, (req) => {
  const segment = (req.headers.get("host") ?? "").split(".")[0];
  return handlers.get(segment)?.(req)
    ?? new Response("unknown host", { status: 404 });
});

Same primitive Cloudflare Workers solves via isolate.fetch() — no
per-build port binding, no per-request localhost loopback.

Implementation

New preset deno-handler (sibling of deno-server / deno-deploy):

• Mirrors deno-server's entry shape minus the Deno.serve() call.
• Exports fetch (named) and default { fetch } — mirrors
  cloudflare-module's convention.
• Drops startScheduleRunner() (library callers own the scheduling
  lifecycle).
• Same rollupConfig, exportConditions, websocket adapter,
  addEventListener error trapping as deno-server.
• No commands.preview / hooks.compiled (embedding host owns
  deno.json).

Verification

test/presets/deno-handler.test.ts boots a minimal Deno.serve wrapper
around the exported fetch and runs the shared testNitro() conformance
suite against it.

43/43 tests pass — identical pass/skip ratios to the deno-server v2
baseline. Skip-list updates in test/tests.ts mirror deno-server's
(same Deno-runtime limitations: module-conflict resolution, sourcemap,
SQL DB driver, BroadcastChannel unstable).

Also verified end-to-end with two real Nitro builds + a routing dispatcher
(multi-site Host-header routing, no port binding at module import) —
matches the production use case the preset is designed for.

Notes

• WebSocket upgrade requires the caller to pass info
  (Deno.ServeHandlerInfo) as the second arg to fetch(request, info);
  throws clearly if missing. Non-WS calls only need the Request.
• Scheduled tasks are deliberately omitted from the library entry —
  the host driver owns scheduling.
• Happy to discuss the design via issue first if preferred; opening this
  as a concrete starting point with passing conformance tests.

[vercel]

@petarbasic is attempting to deploy a commit to the Nitro Team on Vercel.

A member of the Team first needs to authorize it.

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