Only unwrap declared checked causes with UNWRAP

[young0264]

Summary

This updates UNWRAP propagation so that Feign only rethrows:

• RuntimeException and Error
• checked exceptions declared by the target method, including assignable supertypes

Undeclared checked causes now remain wrapped in RetryableException.

Why

Previously, UNWRAP would rethrow any non-null cause from RetryableException.

That could rethrow checked exceptions that are not declared by the target method, which can lead to inconsistent exception propagation at the proxy boundary.

This keeps the fix at the core boundary instead of adding client-specific handling or synthesizing responses.

Tests

Added coverage for:

• undeclared checked causes staying wrapped with UNWRAP
• declared checked causes being unwrapped with UNWRAP
• assignable checked causes being unwrapped with UNWRAP
• the Apache HttpClient redirect-without-Location case staying wrapped as RetryableException

Fixes #1487