feat: introduce and integrate TaskVersion

[yarolegovich]

This is the third refactoring towards multi-process task processor (re #115).

This is a breaking change of a2asrv.TaskStore and eventqueue.Queue. The migration for a single process server is straightforward as a2a.TaskVersionMissing can be returned without affecting any functionality.

• Introduce a2a.TaskVersion interface.
• Extend a2asrv.TaskStore method signatures. Save now takes event which led to Task update and the last known task version.
• Extend eventqueue.Queue method signatures to make task version propagation possible. This will be used for gap and duplicate notification detection when remote executions are added.
• In ./internal/taskexec introduced an exported executionHandler to unify cancelation and execution event processing. The same code is going to be used by remote execution as well.

[gemini-code-assist]

Summary of Changes

Hello @yarolegovich, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request is a significant refactoring step towards enabling a multi-process task processor. It introduces a TaskVersion concept to allow for optimistic concurrency control when updating tasks. Core interfaces like TaskStore and eventqueue.Queue are updated to incorporate this versioning, and the task execution event processing flow is centralized into a new executionHandler to unify logic and pave the way for remote execution scenarios.

Highlights

• Introduce TaskVersion Concept: A new a2a.TaskVersion interface and its TaskVersionInt implementation have been introduced to track the version of a task, crucial for optimistic concurrency control in a multi-process environment.
• Update TaskStore Interface: The a2asrv.TaskStore interface has been updated. Its Save method now accepts the event that led to the task update and the last known task version, returning the new task version. The Get method now also returns the task's version.
• Extend EventQueue Interfaces: The eventqueue.Queue interfaces (Reader and Writer) have been extended to propagate the TaskVersion. A new WriteVersioned method is added to the Writer interface, enabling gap and duplicate notification detection for remote executions.
• Refactor Task Execution Logic: Task execution and cancellation event processing logic has been unified into a new internal/taskexec/execution_handler.go component. This refactoring streamlines event handling and prepares the system for future remote execution capabilities.
• Breaking Changes: This pull request introduces breaking changes to a2asrv.TaskStore and eventqueue.Queue method signatures due to the integration of the TaskVersion concept. Migration for single-process servers is straightforward by returning a2a.TaskVersionMissing.

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[gemini-code-assist]

Code Review

This pull request introduces the TaskVersion concept, a significant and necessary refactoring to support multi-process task processors. The changes are extensive, touching on task storage, event queues, and execution logic. My review focused on the correctness and consistency of the new versioning mechanism. I've identified a critical issue in the version comparison logic and a missed opportunity to implement optimistic locking in the in-memory task store, which is crucial for the feature's goal. Additionally, I've provided suggestions to improve error handling, ensure consistent event propagation, and fix a test case. Overall, this is a solid step towards the goal, and addressing these points will make the implementation more robust and correct.