fix: require NODE_USE_ENV_PROXY for proxy support (#342)

This PR switches proxy support to Node's native env-proxy handling and
makes the required configuration explicit.

## What changed

- fail fast in both `main` and `post` when proxy configuration is
present without `NODE_USE_ENV_PROXY=1`
- document the supported proxy configuration in `README.md`
- add regression tests for the proxy guard in both entrypoints
- keep the existing successful end-to-end coverage and add a smaller
proxy-specific workflow check that enables native proxy support, points
`https_proxy` at an unreachable proxy, and asserts the action fails
- update the test workflow so the same checks also run on pushes to
`beta`

## Proxy configuration

When using `HTTP_PROXY` or `HTTPS_PROXY`, set `NODE_USE_ENV_PROXY=1` on
the action step. If you need bypass rules, set `NO_PROXY` alongside
them.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

post.js

@@ -3,9 +3,15 @@
 import core from "@actions/core";
 
 import { post } from "./lib/post.js";
-import request from "./lib/request.js";
+import request, { ensureNativeProxySupport } from "./lib/request.js";
 
-post(core, request).catch((error) => {
+async function run() {
+  ensureNativeProxySupport();
+
+  return post(core, request);
+}
+
+run().catch((error) => {
   /* c8 ignore next 3 */
   console.error(error);
   core.setFailed(error.message);