Documentation Request: add a clear testing guide for OnchainKit components and hooks

### Is your documentation request related to a problem? Please describe.

### Problem Description
Common testing challenges include:
- mocking wallet connection state and `chainId`
- simulating successful and failed transactions
- testing components that depend on OnchainKit providers
- avoiding flaky tests caused by real network calls

### Proposed Solution

- Add a dedicated “Testing” section to the OnchainKit documentation.
- Describe recommended testing patterns for components and hooks.
- Provide guidance on mocking wallet providers, network state, and transaction results.
- Include at least one concrete example using a common testing stack (e.g., Jest or Vitest with React Testing Library).
- Clearly distinguish between unit tests (mocked) and integration tests (real providers).