fix(ext/node): make `setAutoPadding(false)` a no-op for GCM ciphers (#32290)
## Summary
- GCM is a stream cipher mode that doesn't use block padding. In
Node.js, `setAutoPadding()` is a no-op for GCM modes, but Deno was
throwing `setAutoPadding(false) not supported for Aes128Gcm/Aes256Gcm
yet` when called on GCM deciphers.
- This broke npm packages like `ssh2` that set auto-padding to false by
default.
- Merges the `auto_pad=false` arms into the existing `auto_pad=true`
arms (using `_` wildcard) for both `Aes128Gcm` and `Aes256Gcm` in
`Decipher::final()`, and removes the now-unused error variants.
Closes #29425
Closes #28589
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>