fix(http-server-js): createPolicyChain returns a Promise when policies are present

[Copilot]

Using global policies with the generated router caused TypeError: Cannot read properties of undefined (reading 'catch') because createPolicyChain returned a function that returned undefined instead of a Promise. The generated router calls .catch() on the dispatch result, which fails when no Promise is returned.

Changes

• src/helpers/router.ts: Made applyPolicy return Promise<void>. The base case wraps out(...) in Promise.resolve(). The recursive case uses new Promise with try/catch to handle synchronous policy errors, chaining downstream via .then(resolve, reject). The returned wrapper function now propagates the Promise.
• generated-defs/helpers/router.ts: Regenerated to reflect the source change.
• test/router.test.ts: Added unit tests covering Promise return, policy execution order, error propagation from out, synchronous policy throws, and request forwarding.

// Previously threw: TypeError: Cannot read properties of undefined (reading 'catch')
const policy: Policy = async (ctx, next) => { next(); };
const router = createDemoServiceRouter({...}, { policies: [policy] });

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• telemetry.astro.build
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node /home/REDACTED/work/typespec/typespec/website/node_modules/.bin/../astro/astro.js build node pec/�� tput-dir ../../w--llmstxt sh _modules/.bin/no../../website/src/content/docs/docs/emitters/openapi3/reference --no-emit ture dist/node-gyp-bigenerate-scenarios-summary sed tobu�� s,\\,/,g sh /node_modules/.bin/sh import @typespecnode ture tobuf/reference node (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[http-server-js][Bug]: adding a global policy throws an error</issue_title>
<issue_description>### Describe the bug

When using http-server-js with a custom global policy, an error is thrown.

const policy: Policy = async (ctx, next) => {
  next();
}
const router = createDemoServiceRouter({...}, { policies: [policy] });
// ...

this will always throw TypeError: Cannot read properties of undefined (reading 'catch')

Reproduction

Here is a repo with the reproduction of each case https://github.com/benjlevesque/repro-typespec-issues/blob/main/policy.test.ts

Resolution lead

The bug seems to be in the implementation of createPolicyChain, which does not return a Promise when at least one Policy is specified.
Adding async or returning Promise.resolve fixes the problem

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [http-server-js][Bug]: adding a global policy throws an error #9888

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[benjlevesque]

Considering the result function of createPolicyChain is always awaited, I feel Out should always return a promise, so that the result is necessarily a promise, whether there's a policy or not