fix: propagate suspend result in Redis request invoker

[Keviro]

Motivation

• Ensure coroutine suspension is correctly propagated when invoking suspend request handlers so COROUTINE_SUSPENDED is not dropped and in-flight continuations/responses are not lost.

Description

• Return the result of HANDLE.invoke(context, $completion) in the IS_SUSPEND branch of RedisRequestHandlerInvokerTemplate.invoke(...) so the suspend call's result (e.g., COROUTINE_SUSPENDED) is propagated, while leaving the non-suspend branch unchanged.

Testing

• Attempted to run ./gradlew :surf-redis-core:test, but the build could not run in this environment because the configured Gradle plugin requires Java 25 while the runner JVM is Java 21, so no automated test results were produced.

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Codex Task

[Copilot]

Pull request overview

This PR fixes coroutine suspension propagation for Redis request handler invocation by returning the result of the underlying MethodHandle call in the suspend-path of the hidden-class invoker template, ensuring COROUTINE_SUSPENDED is not dropped during handler dispatch.

Changes:

• Propagate the return value of HANDLE.invoke(context, $completion) for suspend request handlers so coroutine suspension/resumption works correctly.
• Leave non-suspend handler invocation behavior unchanged.