fix(web): surface max-tokens turn ends as a localized truncation notice

A turn the provider ends at its per-request output cap previously produced
no visible sign in the Web chat flow: only error-kind turn/end events built
a conversation node, so the truncated answer read as a normal completion.

Add a turn-max-tokens conversation node Definition over the durable
turn/end event, a warning-styled localized notice row distinct from
turn-error, a fixture max-tokens sample turn, and an assembled keyless
snapshot pinning the dot state, title, and hint.

Closes #1522

packages/client/runtime/src/client/index.ts

@@ -77,7 +77,8 @@ export type {
   CommandNode, CompactionSummaryNode, ComposerPhase,
   ContextMessageNode, ConversationNode, ConversationSnapshot, ModelRetryNode, QueuedMessage,
   LegacyConversationSlice, PartialAssistant, RunningToolCall,
-  SteeringMessageNode, TodoItem, ToolCallBlock, ToolResultNode, TurnErrorNode, UnknownSurfaceNode, UserMessageNode,
+  SteeringMessageNode, TodoItem, ToolCallBlock, ToolResultNode, TurnErrorNode, TurnMaxTokensNode,
+  UnknownSurfaceNode, UserMessageNode,
 } from './sessions/conversation.ts'
 export {
   EMPTY_CHAT_SNAPSHOT, EMPTY_CONVERSATION_VIEWS, toAssistantBlock, toAssistantBlocks,

packages/client/runtime/src/client/sessions/conversation.ts

@@ -163,6 +163,17 @@ export interface TurnErrorNode {
   code?: string
 }
 
+/** Durable notice for a turn ended by the per-request output-token cap. */
+export interface TurnMaxTokensNode {
+  kind: 'turn-max-tokens'
+  /** Seq of the owning turn/end event. */
+  seq: number
+  /** Unix epoch ms from the turn/end event. */
+  time: number
+  turn: number
+  step: number
+}
+
 /** A tool result paired (when in-window) with its call head. */
 export interface ToolResultNode {
   kind: 'tool-result'
@@ -268,6 +279,7 @@ export type ConversationNode =
   | ContextMessageNode
   | ModelRetryNode
   | TurnErrorNode
+  | TurnMaxTokensNode
   | ToolResultNode
   | CommandNode
   | CompactionSummaryNode