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>

main.js

@@ -6,6 +6,7 @@ import { createAppAuth } from "@octokit/auth-app";
 import { getPermissionsFromInputs } from "./lib/get-permissions-from-inputs.js";
 import { main } from "./lib/main.js";
 import request from "./lib/request.js";
+import { runWithProxy } from "./lib/run-with-proxy.js";
 
 if (!process.env.GITHUB_REPOSITORY) {
   throw new Error("GITHUB_REPOSITORY missing, must be set to '<owner>/<repo>'");
@@ -15,32 +16,34 @@ if (!process.env.GITHUB_REPOSITORY_OWNER) {
   throw new Error("GITHUB_REPOSITORY_OWNER missing, must be set to '<owner>'");
 }
 
-const appId = core.getInput("app-id");
-const privateKey = core.getInput("private-key");
-const owner = core.getInput("owner");
-const repositories = core
-  .getInput("repositories")
-  .split(/[\n,]+/)
-  .map((s) => s.trim())
-  .filter((x) => x !== "");
-
-const skipTokenRevoke = core.getBooleanInput("skip-token-revoke");
-
-const permissions = getPermissionsFromInputs(process.env);
-
 // Export promise for testing
-export default main(
-  appId,
-  privateKey,
-  owner,
-  repositories,
-  permissions,
-  core,
-  createAppAuth,
-  request,
-  skipTokenRevoke,
-).catch((error) => {
-  /* c8 ignore next 3 */
-  console.error(error);
-  core.setFailed(error.message);
+export default runWithProxy(async () => {
+  const appId = core.getInput("app-id");
+  const privateKey = core.getInput("private-key");
+  const owner = core.getInput("owner");
+  const repositories = core
+    .getInput("repositories")
+    .split(/[\n,]+/)
+    .map((s) => s.trim())
+    .filter((x) => x !== "");
+
+  const skipTokenRevoke = core.getBooleanInput("skip-token-revoke");
+
+  const permissions = getPermissionsFromInputs(process.env);
+
+  return main(
+    appId,
+    privateKey,
+    owner,
+    repositories,
+    permissions,
+    core,
+    createAppAuth,
+    request,
+    skipTokenRevoke,
+  ).catch((error) => {
+    /* c8 ignore next 3 */
+    console.error(error);
+    core.setFailed(error.message);
+  });
 });