fix: raise API errors as exceptions instead of text messages (#472)

[dicksontsai]

Summary

• Add exception classes for API errors (400, 401, 429, 529, etc.)
• Raise exceptions instead of returning errors as text messages
• Enable programmatic error handling

Problem

API errors from the Anthropic API were being returned as text messages in the chat stream, making it impossible to:

• Implement retry logic for transient errors (529 Overloaded)
• Detect authentication failures programmatically
• Handle rate limiting (429) with backoff

Solution

1. Added new exception classes:

   • APIError (base class)
   • AuthenticationError (for 401)
   • BillingError (for billing issues)
   • RateLimitError (for 429)
   • InvalidRequestError (for 400)
   • ServerError (for 500, 529)

2. Check AssistantMessage.error field and raise appropriate exceptions

Usage

from claude_agent_sdk import query, RateLimitError, InvalidRequestError

try:
    async for msg in query(prompt="test"):
        ...
except RateLimitError:
    # Implement retry logic
except InvalidRequestError as e:
    print(f"Invalid request: {e.error_text}")

Test plan

• Added tests for all new exception classes
• Added tests for error raising in client
• All existing tests pass (131 tests)
• Linting passes

Closes #472