CONTRIBUTING.md references removed testlocal:e2e script

**Bicep version**

Contributor documentation on `main`, checked at commit `791498a765ae3a1b7407f38330a3d44d0c7f77cf`.

**Describe the bug**

The [contribution guide](https://github.com/Azure/bicep/blob/791498a765ae3a1b7407f38330a3d44d0c7f77cf/CONTRIBUTING.md#running-the-tests) instructs contributors to run `npm run testlocal:e2e` to execute the VS Code extension’s end-to-end tests.

However, `testlocal:e2e` is no longer defined in the extension’s [package.json](https://github.com/Azure/bicep/blob/791498a765ae3a1b7407f38330a3d44d0c7f77cf/src/vscode-bicep/package.json), so the documented command cannot start the tests.

**To Reproduce**

1. Check out the revision above.
2. Open a terminal in `src/vscode-bicep`.
3. Run:

```sh
npm run testlocal:e2e
```

The script is missing from the package’s available commands.

**Expected behavior**

The contribution guide should describe the supported local E2E workflow:

```sh
node ./scripts/run-e2e-tests.mjs
```

This command is already documented in the [extension E2E guide](https://github.com/Azure/bicep/blob/791498a765ae3a1b7407f38330a3d44d0c7f77cf/src/vscode-bicep/tests/e2e/README.md#commands).

**Additional context**

The old script was removed in [PR #20200](https://github.com/Azure/bicep/pull/20200), merged on 19 August 2026, when the extension tests were migrated to Vitest.