`config.d.ts` union type precedence bug causes config validation failure for `dynatrace.environments`

# Summary

The TypeScript union type definition for `environments` in `plugins/dql-backend/config.d.ts` has incorrect operator precedence, causing Backstage's config schema compiler to generate an `object` type instead of an `array` type. This makes the app fail to start with a config validation error when using the documented array format for `dynatrace.environments`.

## Expected Behavior

Backstage should start successfully when `dynatrace.environments` is configured as an array, as shown in the plugin's own README documentation:

```yaml
dynatrace:
environments:
- name: xxxxxxxx
url: https://xxxxxxxx.apps.dynatrace.com
tokenUrl: https://sso.dynatrace.com/sso/oauth2/token
accountUrn:
clientId:
clientSecret:
```

## Current Behavior

Backstage fails to start with the following error:

```
Plugin 'app' threw an error during startup. Invalid app bundle schema.
Caused by: Config validation failed, Config must be object { type=object } at /dynatrace/environments
```

## Steps to Reproduce

1. Install `@dynatrace/backstage-plugin-dql-backend` v2.6.0
2. Configure `dynatrace.environments` as an array in `app-config.yaml` (following the README)
3. Build the app with `yarn --cwd packages/app build`
4. Start the backend

## Possible Solution

The issue is in `plugins/dql-backend/config.d.ts` 19. The current type:

```typescript
environments:
{ /* OAuthConfig */ } | { /* TokenConfig */ }[],
```

Due to TypeScript operator precedence, this is parsed as `OAuthConfig | (TokenConfig[])` — a union of a single object or an array. Backstage's config schema compiler picks the first branch (object) and generates `{ type: "object" }` for the JSON schema.

The fix is to wrap the union in parentheses so the array applies to both types:

```typescript
environments:
({ /* OAuthConfig */ } | { /* TokenConfig */ })[],
```

This makes it unambiguously `(OAuthConfig | TokenConfig)[]` — an array of either type, which matches the documented config format.

## Context

This prevents deploying Backstage with the Dynatrace DQL plugin in any environment where the app bundle is built.
The error occurs in `@backstage/plugin-app-backend` at `readFrontendConfig.cjs.js:26` during config schema validation against the compiled app bundle.

## Your Environment

- `@dynatrace/backstage-plugin-dql-backend` version: 2.6.0
- `@dynatrace/backstage-plugin-dql` version: 2.6.0