Propagate request attribution into Servlet async worker execution

[TaeHuiKKIM]

Problem

The MVC interceptor preserves attribution on async re-dispatch, but code executed inside a Callable or DeferredResult worker before re-dispatch is not automatically attached to the originating request bucket.

Proposed scope

• evaluate CallableProcessingInterceptor and DeferredResultProcessingInterceptor integration
• restore and clear the Reqover bucket around worker execution
• preserve the fail-closed rule: missing context should become unattributed, never attached to another request
• add concurrent integration tests for Callable and DeferredResult

Acceptance criteria

• worker-thread method-entry hits appear under the originating normalized endpoint
• two overlapping async requests remain isolated
• context is cleared on success, timeout, and error
• the limitation in README and SECURITY/integration docs is updated