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>

tests/index.js

@@ -21,6 +21,14 @@ for (const file of testFiles) {
     const env = {
       GITHUB_OUTPUT: undefined,
       GITHUB_STATE: undefined,
+      HTTP_PROXY: undefined,
+      HTTPS_PROXY: undefined,
+      http_proxy: undefined,
+      https_proxy: undefined,
+      NO_PROXY: undefined,
+      no_proxy: undefined,
+      NODE_OPTIONS: undefined,
+      NODE_USE_ENV_PROXY: undefined,
     };
     const { stderr, stdout } = await execa("node", [`tests/${file}`], { env });
     t.snapshot(stderr, "stderr");

tests/main-proxy-requires-native-support.test.js

@@ -0,0 +1,14 @@
+process.env.GITHUB_REPOSITORY = "actions/create-github-app-token";
+process.env.GITHUB_REPOSITORY_OWNER = "actions";
+process.env.HTTPS_PROXY = "http://127.0.0.1:3128";
+
+const originalConsoleError = console.error;
+console.error = (...args) => {
+  originalConsoleError(
+    ...args.map((arg) => (arg instanceof Error ? arg.message : arg)),
+  );
+};
+
+await import("../main.js");
+await new Promise((resolve) => setImmediate(resolve));
+process.exitCode = 0;

tests/post-proxy-requires-native-support.test.js

@@ -0,0 +1,13 @@
+process.env["INPUT_GITHUB-API-URL"] = "https://api.github.com";
+process.env.HTTPS_PROXY = "http://127.0.0.1:3128";
+
+const originalConsoleError = console.error;
+console.error = (...args) => {
+  originalConsoleError(
+    ...args.map((arg) => (arg instanceof Error ? arg.message : arg)),
+  );
+};
+
+await import("../post.js");
+await new Promise((resolve) => setImmediate(resolve));
+process.exitCode = 0;

tests/snapshots/index.js.md

@@ -82,6 +82,16 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"]}`
 
+## main-proxy-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## main-repo-skew.test.js
 
 > stderr
@@ -333,6 +343,16 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"],"permissions":{"issues":"write","pull_requests":"read"}}`
 
+## post-proxy-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## post-revoke-token-fail-response.test.js
 
 > stderr