fix: add 429 rate limit retry with exponential backoff

[xodn348]

Summary

Closes #812

This PR adds automatic retry logic for 429 rate limit errors in the Claude Agent SDK Python client.

Changes

• ****: New RateLimitError exception class with retry_after and original_error fields
• ****: Export RateLimitError from public API
• ****: Add rate_limit_max_retries field to ClaudeAgentOptions (default: 3)
• ****: Retry loop with exponential backoff in process_query()
• ****: 14 tests covering all retry scenarios

Retry Behavior

• Detects 429 errors by inspecting error messages and stderr for rate_limit_error or 429
• Parses Retry-After / retryAfter header to respect server-specified wait time
• Falls back to exponential backoff: min(2.0 * 2^(attempt-1), 60.0) + jitter
• Respects rate_limit_max_retries option (default 3); raises RateLimitError after exhaustion
• Non-rate-limit errors propagate immediately without retry

Testing

pytest tests/test_rate_limit_retry.py -v
# 14 passed in 0.27s