fix: parse error field in AssistantMessage to enable rate limit detection (#405)

## Summary

Fixes #401

Enables applications to detect API errors (especially `rate_limit`
errors) by properly parsing the `error` field in `AssistantMessage`.

## Problem

The SDK defines `AssistantMessage.error` (including `"rate_limit"`), but
the message parser never extracted this field from the CLI response.
This made it impossible for applications to:
- Detect when rate limits are hit
- Implement retry logic
- Handle other API errors gracefully

## Solution

Added error field extraction in the message parser:

```python
return AssistantMessage(
    content=content_blocks,
    model=data["message"]["model"],
    parent_tool_use_id=data.get("parent_tool_use_id"),
    error=data["message"].get("error"),  # ← Now extracts error field
)
```

## Changes

**Modified: `src/claude_agent_sdk/_internal/message_parser.py`**

The parser now extracts the `error` field from API responses and
populates it in the `AssistantMessage` object.

## Usage Example

Applications can now detect and handle rate limits:

```python
async for message in client.receive_response():
    if isinstance(message, AssistantMessage):
        if message.error == "rate_limit":
            print("Rate limit hit! Implementing backoff...")
            await asyncio.sleep(60)
            # Retry logic here
        elif message.error:
            print(f"API error: {message.error}")
```

## Testing

- ✅ Passed ruff linting and formatting
- ✅ Passed mypy type checking
- ✅ All existing tests pass

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

## Impact

This fix enables production applications to:
- Implement proper error handling for API errors
- Build robust retry logic for rate limits
- Provide better user feedback when errors occur
- Avoid silent failures when the API returns errors

---

🤖 Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <[email protected]>
Co-Authored-By: Happy <[email protected]>

Co-authored-by: Claude <[email protected]>
Co-authored-by: Happy <[email protected]>

Author: 3 people

src/claude_agent_sdk/_internal/message_parser.py

@@ -120,6 +120,7 @@ def parse_message(data: dict[str, Any]) -> Message:
                     content=content_blocks,
                     model=data["message"]["model"],
                     parent_tool_use_id=data.get("parent_tool_use_id"),
+                    error=data["message"].get("error"),
                 )
             except KeyError as e:
                 raise MessageParseError(