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

[Copilot]

createPolicyChain discarded the return value of applyPolicy, causing the returned function to return undefined instead of a Promise. The generated router calls .catch() on dispatch(...), which threw TypeError: Cannot read properties of undefined (reading 'catch') whenever any policy was configured.

Changes

• src/helpers/router.ts: Made applyPolicy return Promise<void>. Base case wraps out(...) in Promise.resolve() for proper error propagation. Recursive case uses new Promise with try/catch for synchronous policy errors, chaining downstream via .then(resolve, reject). The returned function now returns the promise from applyPolicy.
• generated-defs/helpers/router.ts: Regenerated to reflect the source change.
• test/router.test.ts: Added 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] });

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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