Tracking: Migrate a2a-server to event-driven Scheduler

[cocosheng-g]

Description

This issue tracks the work to migrate packages/a2a-server to use the modern, event-driven Scheduler by default, replacing the legacy callback-based CoreToolScheduler. The PR transitioning A2A-server to the message-bus based tool execution has been consolidated under #21467, superseding the original #17128.

Tasks

• Migrate a2a-server internal tools to event-driven execution via MessageBus.
• Integrate correlationId tracking for interactive confirmations.
• Resolve rebase merge conflicts from recent upstream changes.
• Refactor large closures on MessageBus subscriptions and clean up type assertions.