#1519 Avoid unused delegate field in decorators - #4099

Fixes #1519.

This avoids generating the private delegate field and its constructor assignment when the decorator implements all mapper methods itself.

Constructor generation now handles both decorator shapes:

• A user-defined delegate constructor still receives the generated delegate through super(delegate).
• A default-constructor decorator no longer creates an unused delegate instance or an empty private delegate constructor.

The change also avoids retaining unused mapper imports for custom implementation packages, including nested decorators, while preserving imports required by generated annotations.

Regression coverage runs with both javac and Eclipse compilers and covers delegate-constructor and default-constructor decorators. Existing partial-decorator, Spring, JSR-330/Jakarta, cross-package, and destination-package suites remain green.

Tests:

• Scoped decorator/component-model verify: 13 test classes, including Spring constructor/setter/field, JSR-330/Jakarta, cross-package, destination-package, and issues Very minor suggestion: don't generate private delegate field if it's not used #1519/Wrong decorator class name used when Decorator class is nested inside mapper interface #2021
• Full processor suite: 3,639 tests, 0 failures, 0 errors
• forbiddenapis: 661 classes, 0 errors