feat: add typed RateLimitEvent message

[qing-ant]

Summary

Adds typed parsing for rate_limit_event messages emitted by the CLI when rate limit status changes for claude.ai subscription users. After #598 stopped the parser from crashing on unknown types, these events were silently dropped — users couldn't warn before hitting limits or back off gracefully when rejected.

What's added

• RateLimitEvent dataclass with rate_limit_info, uuid, session_id
• RateLimitInfo dataclass with status, resets_at, rate_limit_type, utilization, overage_status, overage_resets_at, overage_disabled_reason, raw
• RateLimitStatus ("allowed" | "allowed_warning" | "rejected") and RateLimitType literal types
• Parser case for rate_limit_event message type
• Exports for all the above plus StreamEvent (was in Message union but missing from __init__.py)

Field names follow Python conventions (snake_case); camelCase CLI fields are mapped in the parser. The raw field preserves the original dict including any unmodeled fields for forward compatibility.

E2E verification

Captured a real event from the CLI:

status: allowed_warning
resets_at: 1773273600
rate_limit_type: seven_day
utilization: 0.62
raw keys: ['isUsingOverage', 'rateLimitType', 'resetsAt', 'status', 'utilization']

Usage

from claude_agent_sdk import query, RateLimitEvent

async for msg in query(prompt="..."):
    if isinstance(msg, RateLimitEvent):
        if msg.rate_limit_info.status == "allowed_warning":
            print(f"Approaching limit, resets at {msg.rate_limit_info.resets_at}")
        elif msg.rate_limit_info.status == "rejected":
            print("Rate limited — backing off")

Fixes #583, #599, #601, #603