Provide structured error categories and handling helpers for agent actions

### Language Implementation

- [ ] Python
- [ ] TypeScript

### Feature Type

- [ ] Action Provider Template
- [ ] Wallet Provider Template
- [ ] Framework Extension
- [ ] Core Requirements
- [ ] Other

### 🚀 The feature, motivation and pitch

## Overview
AgentKit exposes various errors during agent execution (validation failures, network issues, retries, terminal errors), but developers currently need to infer how to classify and handle these errors themselves. A first-class, structured error model would improve consistency and developer experience.

## Problem Description
Without standardized error categories:
- Developers treat all errors uniformly, even when behavior should differ.
- Retryable errors and misconfiguration errors are hard to distinguish.
- UX and logging logic becomes inconsistent across applications.

## Proposed Solution
Introduce structured error categories (e.g. validation, retryable, terminal) and lightweight helpers or metadata that allow developers to handle errors predictably without inspecting internal error shapes.

### Acceptance Criteria
Developers can reliably distinguish between retryable and non-retryable errors. Error handling becomes more consistent across AgentKit integrations. The change is additive and does not break existing error handling logic.

### Alternatives

_No response_

### Additional context

_No response_