fix: resolve false-positive rate limit and one-message-behind in chat 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>

ui/src/hooks/useAssistantChat.ts

@@ -269,20 +269,6 @@ export function useAssistantChat({
             break;
           }
 
-          case "rate_limited": {
-            // Show rate limit info as system message
-            setMessages((prev) => [
-              ...prev,
-              {
-                id: generateId(),
-                role: "system",
-                content: `Rate limited. Retrying in ${data.retry_in}s... (attempt ${data.attempt}/${data.max_attempts})`,
-                timestamp: new Date(),
-              },
-            ]);
-            break;
-          }
-
           case "pong": {
             // Keep-alive response, nothing to do
             break;

ui/src/hooks/useExpandChat.ts

@@ -226,20 +226,6 @@ export function useExpandChat({
             break
           }
 
-          case 'rate_limited': {
-            // Show rate limit info as system message
-            setMessages((prev) => [
-              ...prev,
-              {
-                id: generateId(),
-                role: 'system',
-                content: `Rate limited. Retrying in ${data.retry_in}s... (attempt ${data.attempt}/${data.max_attempts})`,
-                timestamp: new Date(),
-              },
-            ])
-            break
-          }
-
           case 'pong': {
             // Keep-alive response, nothing to do
             break

ui/src/hooks/useSpecChat.ts

@@ -322,20 +322,6 @@ export function useSpecChat({
             break
           }
 
-          case 'rate_limited': {
-            // Show rate limit info as system message
-            setMessages((prev) => [
-              ...prev,
-              {
-                id: generateId(),
-                role: 'system',
-                content: `Rate limited. Retrying in ${data.retry_in}s... (attempt ${data.attempt}/${data.max_attempts})`,
-                timestamp: new Date(),
-              },
-            ])
-            break
-          }
-
           case 'pong': {
             // Keep-alive response, nothing to do
             break

ui/src/lib/types.ts

@@ -407,13 +407,6 @@ export interface SpecChatResponseDoneMessage {
   type: 'response_done'
 }
 
-export interface SpecChatRateLimitedMessage {
-  type: 'rate_limited'
-  retry_in: number
-  attempt: number
-  max_attempts: number
-}
-
 export type SpecChatServerMessage =
   | SpecChatTextMessage
   | SpecChatQuestionMessage
@@ -423,7 +416,6 @@ export type SpecChatServerMessage =
   | SpecChatErrorMessage
   | SpecChatPongMessage
   | SpecChatResponseDoneMessage
-  | SpecChatRateLimitedMessage
 
 // Image attachment for chat messages
 export interface ImageAttachment {
@@ -509,13 +501,6 @@ export interface AssistantChatPongMessage {
   type: 'pong'
 }
 
-export interface AssistantChatRateLimitedMessage {
-  type: 'rate_limited'
-  retry_in: number
-  attempt: number
-  max_attempts: number
-}
-
 export type AssistantChatServerMessage =
   | AssistantChatTextMessage
   | AssistantChatToolCallMessage
@@ -524,7 +509,6 @@ export type AssistantChatServerMessage =
   | AssistantChatErrorMessage
   | AssistantChatConversationCreatedMessage
   | AssistantChatPongMessage
-  | AssistantChatRateLimitedMessage
 
 // ============================================================================
 // Expand Chat Types
@@ -548,7 +532,6 @@ export type ExpandChatServerMessage =
   | SpecChatErrorMessage       // Reuse error message type
   | SpecChatPongMessage        // Reuse pong message type
   | SpecChatResponseDoneMessage // Reuse response_done type
-  | SpecChatRateLimitedMessage // Reuse rate_limited message type
 
 // Bulk feature creation
 export interface FeatureBulkCreate {