fix: handle rate_limit_event crash in chat sessions

The Claude CLI sends `rate_limit_event` messages that the SDK's
`parse_message()` doesn't recognize, raising `MessageParseError` and
crashing all three chat session types (spec, assistant, expand).

Changes:
- Bump claude-agent-sdk minimum from 0.1.0 to 0.1.39
- Add `check_rate_limit_error()` helper in chat_constants.py that
  detects rate limits from both MessageParseError data payloads and
  error message text patterns
- Wrap `receive_response()` loops in all three `_query_claude()` methods
  with retry-on-rate-limit logic (up to 3 retries with backoff)
- Gracefully log and skip non-rate-limit MessageParseError instead of
  crashing the session
- Add `rate_limited` message type to frontend TypeScript types and
  handle it in useSpecChat, useAssistantChat, useExpandChat hooks to
  show "Rate limited. Retrying in Xs..." system messages

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

ui/src/hooks/useAssistantChat.ts

@@ -269,6 +269,20 @@ 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

@@ -226,6 +226,20 @@ 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

@@ -322,6 +322,20 @@ 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

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