feat: add global fallback support for AspectJ annotation extension (#3110)

[EvanYao826]

Fixes #3110

Summary

Add SentinelAnnotationGlobalFallback interface and integrate it into AbstractSentinelAspectSupport as a last-resort fallback handler.

When set on SentinelResourceAspect, the global fallback is invoked for all @SentinelResource-annotated methods when neither per-method fallback nor defaultFallback is configured (or found). This provides a universal 兜底 strategy without requiring each annotation to specify its own fallback.

Design

Follows the direction suggested by @sczyh30 in #3116: the global fallback lives in SentinelResourceAspect (not in the annotation), making it a cross-cutting concern.

Fallback resolution order:

1. Per-method fallback (annotation attribute)
2. Per-method defaultFallback (annotation attribute or class-level)
3. Global fallback (set on the aspect) ← NEW
4. Throw exception

Changes

New: SentinelAnnotationGlobalFallback interface

public interface SentinelAnnotationGlobalFallback {
    Object handle(Method originalMethod, Object[] args, Throwable t) throws Throwable;
}

Modified: AbstractSentinelAspectSupport

• Added globalFallback field with getter/setter
• Modified handleDefaultFallback() to try global fallback before throwing

Tests

3 unit tests for get/set/clear global fallback on AbstractSentinelAspectSupportTest.

Usage

@Bean
public SentinelResourceAspect sentinelResourceAspect() {
    SentinelResourceAspect aspect = new SentinelResourceAspect();
    aspect.setGlobalFallback((method, args, t) -> {
        // Universal fallback logic
        return "Global fallback: " + t.getMessage();
    });
    return aspect;
}

Notes

• Previous PRs Add global fallback support for AspectJ annotation extension #3116 and Add global fallback support for AspectJ annotation extension #3383 were closed without merge. This implementation follows the maintainer feedback: global fallback is in the aspect, not the annotation.
• The globalFallback field is volatile for thread-safe reads in the aspect's @Around advice.

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