fix: raise RateLimitError on HTTP 429 instead of crashing

[ashishpatel26]

Summary

Resolves #812

When the Anthropic API returns HTTP 429, the SDK surfaced a generic uncaught exception with no actionable type. This PR adds a typed RateLimitError so callers can catch and handle rate-limit failures separately.

Changes Made

• src/claude_agent_sdk/_errors.py: add RateLimitError(ClaudeSDKError) with optional retry_after: int field
• src/claude_agent_sdk/_internal/query.py: add _is_rate_limit_error() helper; detect 429/rate-limit text in the error path and raise RateLimitError instead of a generic Exception
• src/claude_agent_sdk/__init__.py: export RateLimitError
• tests/test_errors.py: 18 tests covering subclassing, retry_after, and all _is_rate_limit_error detection patterns

Usage After This Fix

from claude_agent_sdk import query, RateLimitError

try:
    async for msg in query("...", options=options):
        ...
except RateLimitError as e:
    print(f"Rate limited. Retry after: {e.retry_after}s")

Testing

• 18 unit tests added in tests/test_errors.py
• All existing tests pass

Related Issue

Closes #812