Shakapacker has environment variable leak via EnvironmentPlugin that exposes secrets to client-side bundles

Summary

### Summary

Since 2017, the default webpack plugins have passed the entire `process.env` to `EnvironmentPlugin`. This pattern exposed ALL build environment variables to client-side JavaScript bundles whenever application code (or any dependency) referenced `process.env.VARIABLE_NAME`.

This is not a regression - the vulnerable code has existed since the original Webpacker implementation. No recent code change in Shakapacker triggered this issue.

### Impact

Any environment variable in the build environment that is referenced in client-side code (including third-party dependencies) is embedded directly into the JavaScript bundle. This includes:

- `DATABASE_URL` - Database credentials - `AWS_SECRET_ACCESS_KEY` - AWS credentials - `RAILS_MASTER_KEY` - Rails encrypted credentials key - `STRIPE_SECRET_KEY`, `TWILIO_AUTH_TOKEN` - Third-party API keys - Any other secrets present in the build environment

**Severity**: Critical - secrets are exposed in publicly accessible JavaScript files.

### Root Cause

The original code used: ```javascript new webpack.EnvironmentPlugin(process.env) ```

This makes every environment variable available for substitution. If any code references `process.env.SECRET_KEY`, that value is embedded in the bundle.

### Patches

Upgrade to version 9.5.0 or later, which uses an allowlist approach that only exposes `NODE_ENV`, `RAILS_ENV`, and `WEBPACK_SERVE` by default.

### Workarounds

If developers cannot upgrade immediately: 1. Audit client-side code and dependencies for any `process.env.X` references to sensitive variables 2. Remove sensitive variables from the build environment 3. Override the default plugins with a custom webpack/rspack config using an explicit allowlist

### Migration

After upgrading, if client-side code needs access to specific environment variables:

**Option 1: Use the `SHAKAPACKER_PUBLIC_` prefix (recommended)** ```bash # Variables with this prefix are automatically exposed export SHAKAPACKER_PUBLIC_API_URL="https://api.example.com" ```

**Option 2: Use `SHAKAPACKER_ENV_VARS`** ```bash SHAKAPACKER_ENV_VARS=API_URL,FEATURE_FLAG bundle exec rails assets:precompile ```

### Action Required

After upgrading, **rotate any secrets** that may have been exposed in previously compiled JavaScript bundles.

### Resources

- Fix PR: https://github.com/shakacode/shakapacker/pull/857