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>

README.md

@@ -296,6 +296,24 @@ jobs:
           GITHUB_TOKEN: ${{ steps.create_token.outputs.token }}
 ```
 
+### Proxy support
+
+This action relies on Node.js native proxy support.
+
+If you set `HTTP_PROXY` or `HTTPS_PROXY`, also set `NODE_USE_ENV_PROXY: "1"` on the action step so Node.js honors those variables. If you need proxy bypass rules, set `NO_PROXY` alongside them.
+
+```yaml
+- uses: actions/create-github-app-token@v3
+  id: app-token
+  env:
+    HTTPS_PROXY: http://proxy.example.com:8080
+    NO_PROXY: github.example.com
+    NODE_USE_ENV_PROXY: "1"
+  with:
+    app-id: ${{ vars.APP_ID }}
+    private-key: ${{ secrets.PRIVATE_KEY }}
+```
+
 ## Inputs
 
 ### `app-id`