feat: add proxy support via child process spawning

When proxy environment variables (https_proxy, HTTPS_PROXY, http_proxy,
HTTP_PROXY) are detected and NODE_USE_ENV_PROXY is not already set to
"1", the action spawns a child process with NODE_USE_ENV_PROXY=1 to
enable Node.js native proxy support.

- Add lib/run-with-proxy.js shared utility for both main.js and post.js
- Update main.js and post.js to use runWithProxy() wrapper
- Add tests for proxy spawning, child error handling, and already-enabled path
- 100% code coverage maintained

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

post.js

@@ -4,9 +4,13 @@ import core from "@actions/core";
 
 import { post } from "./lib/post.js";
 import request from "./lib/request.js";
+import { runWithProxy } from "./lib/run-with-proxy.js";
 
-post(core, request).catch((error) => {
-  /* c8 ignore next 3 */
-  console.error(error);
-  core.setFailed(error.message);
+// Export promise for testing
+export default runWithProxy(async () => {
+  return post(core, request).catch((error) => {
+    /* c8 ignore next 3 */
+    console.error(error);
+    core.setFailed(error.message);
+  });
 });