feat: add grace retry for empty assistant messages (#10297)

Implements grace retry error handling for 'no assistant messages' API
errors, following the same pattern as PR #10196 for 'no tools used'.

- Add consecutiveNoAssistantMessagesCount counter to Task.ts
- First failure: silent retry (grace retry)
- After 2+ consecutive failures: show MODEL_NO_ASSISTANT_MESSAGES error
- Add UI handling in ChatRow.tsx with ErrorRow component
- Add localized strings to all 18 locale files
- Add comprehensive tests for the grace retry behavior

Author: hannesrudolph

src/core/task/Task.ts

@@ -311,6 +311,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 	consecutiveMistakeLimit: number
 	consecutiveMistakeCountForApplyDiff: Map<string, number> = new Map()
 	consecutiveNoToolUseCount: number = 0
+	consecutiveNoAssistantMessagesCount: number = 0
 	toolUsage: ToolUsage = {}
 
 	// Checkpoints
@@ -1985,6 +1986,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 		// Reset consecutive error counters on abort (manual intervention)
 		this.consecutiveNoToolUseCount = 0
+		this.consecutiveNoAssistantMessagesCount = 0
 
 		// Force final token usage update before abort event
 		this.emitFinalTokenUsageUpdate()
@@ -3158,6 +3160,8 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				)
 
 				if (hasTextContent || hasToolUses) {
+					// Reset counter when we get a successful response with content
+					this.consecutiveNoAssistantMessagesCount = 0
 					// Display grounding sources to the user if they exist
 					if (pendingGroundingSources.length > 0) {
 						const citationLinks = pendingGroundingSources.map((source, i) => `[${i + 1}](${source.url})`)
@@ -3291,6 +3295,15 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 					// or tool_use content blocks from API which we should assume is
 					// an error.
 
+					// Increment consecutive no-assistant-messages counter
+					this.consecutiveNoAssistantMessagesCount++
+
+					// Only show error and count toward mistake limit after 2 consecutive failures
+					// This provides a "grace retry" - first failure retries silently
+					if (this.consecutiveNoAssistantMessagesCount >= 2) {
+						await this.say("error", "MODEL_NO_ASSISTANT_MESSAGES")
+					}
+
 					// IMPORTANT: For native tool protocol, we already added the user message to
 					// apiConversationHistory at line 1876. Since the assistant failed to respond,
 					// we need to remove that message before retrying to avoid having two consecutive