… sessions

The Claude Code CLI v2.1.45+ emits a `rate_limit_event` message type that
the Python SDK v0.1.19 cannot parse, raising MessageParseError. Two bugs
resulted:

1. **False-positive rate limit**: check_rate_limit_error() matched
   "rate_limit" in the exception string "Unknown message type:
   rate_limit_event" via both an explicit type check and a regex fallback,
   triggering 15-19s backoff + query re-send on every session.

2. **One-message-behind**: The MessageParseError killed the
   receive_response() async generator, but the CLI subprocess was still
   alive with buffered response data. Catching and returning meant the
   response was never consumed. The next send_message() would read the
   previous response first, creating a one-behind offset.

Changes:

- chat_constants.py: check_rate_limit_error() now returns (False, None)
  for any MessageParseError, blocking both false-positive paths. Added
  safe_receive_response() helper that retries receive_response() on
  MessageParseError — the SDK's decoupled producer/consumer architecture
  (anyio memory channel) allows the new generator to continue reading
  remaining messages without data loss. Removed calculate_rate_limit_backoff
  re-export and MAX_CHAT_RATE_LIMIT_RETRIES constant.

- spec_chat_session.py, assistant_chat_session.py, expand_chat_session.py:
  Replaced retry-with-backoff loops with safe_receive_response() wrapper.
  Removed asyncio.sleep backoff, query re-send, and rate_limited yield.
  Cleaned up unused imports (asyncio, calculate_rate_limit_backoff,
  MAX_CHAT_RATE_LIMIT_RETRIES).

- agent.py: Added inner retry loop around receive_response() with same
  MessageParseError skip-and-restart pattern. Removed early-return that
  truncated responses.

- types.ts: Removed SpecChatRateLimitedMessage,
  AssistantChatRateLimitedMessage, and their union entries.

- useSpecChat.ts, useAssistantChat.ts, useExpandChat.ts: Removed dead
  'rate_limited' case handlers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>