Preserve endpointFetch response headers

[akalinin-stripe]

Why?

EndpointFetchHttpClient currently replaces response headers with an empty object. This prevents response metadata and retry signals returned by endpointFetch from reaching stripe-node responses and errors.

What?

• Preserve optional response headers from successful endpointFetch calls.
• Preserve optional response headers from response-shaped rejections.
• Keep the existing empty-header behavior when the runtime omits headers.
• Add coverage for SDK response headers and retry-after on Stripe API errors.

Test Plan

• tsc -p tsconfig.cjs.json --pretty false
• mocha test/net/EndpointFetchHttpClient.spec.ts test/ExtensibilityPlatformFunctions.spec.ts
• Targeted ESLint and Prettier checks for changed files

Changelog

• Preserves response headers returned by endpointFetch in extensibility runtimes.