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/ui-conversation/tests/conversation-node-definitions.client.spec.ts

@@ -14,6 +14,7 @@ import { messageDefinition } from '../src/client/conversation-nodes/message.ts'
 import { retryDefinition } from '../src/client/conversation-nodes/retry.ts'
 import { toolDefinition } from '../src/client/conversation-nodes/tool.ts'
 import { turnErrorDefinition } from '../src/client/conversation-nodes/turn-error.ts'
+import { turnMaxTokensDefinition } from '../src/client/conversation-nodes/turn-max-tokens.ts'
 import { turnTailDefinition } from '../src/client/conversation-nodes/turn-tail.ts'
 import type {
   AssistantChatData, ManualCompactionChatData, RetryChatData, ToolChatData, TurnTailChatData,
@@ -29,6 +30,7 @@ const DEFINITIONS: readonly ConversationNodeDefinition[] = [
   compactionDefinition,
   retryDefinition,
   turnErrorDefinition,
+  turnMaxTokensDefinition,
   turnTailDefinition,
 ]
 
@@ -812,6 +814,74 @@ describe('built-in conversation node Definitions', () => {
     expect(node(snapshot(value), 'turn-error')).toBeUndefined()
   })
 
+  it('materializes a max-tokens notice and keeps completed and error turns clean', () => {
+    const value = assembler([
+      at(1, 'turn/start', { turn: 1 }),
+      at(2, 'step/start', { turn: 1, step: 1 }),
+      at(3, 'assistant/message', {
+        turn: 1, step: 1, message: assistantMessage('a1', 'truncated answer'),
+      }, { surfaceOp: 'append' }),
+      at(4, 'step/end', { turn: 1, step: 1 }),
+      at(5, 'turn/end', { turn: 1, reason: { kind: 'max-tokens' } }),
+    ])
+    const notice = node(snapshot(value), 'turn-max-tokens')
+    expect(notice?.data).toMatchObject({ kind: 'turn-max-tokens', seq: 5, turn: 1, step: 1 })
+    expect(node(snapshot(value), 'turn-error')).toBeUndefined()
+
+    const completed = assembler([
+      at(1, 'turn/start', { turn: 1 }),
+      at(2, 'turn/end', { turn: 1, reason: { kind: 'completed' } }),
+    ])
+    expect(node(snapshot(completed), 'turn-max-tokens')).toBeUndefined()
+
+    const failed = assembler([
+      at(1, 'turn/start', { turn: 1 }),
+      at(2, 'turn/end', {
+        turn: 1,
+        reason: { kind: 'error', error: { code: 'TRANSPORT', message: 'failed' } },
+      }),
+    ])
+    expect(node(snapshot(failed), 'turn-max-tokens')).toBeUndefined()
+    expect(node(snapshot(failed), 'turn-error')).toBeDefined()
+  })
+
+  it('keeps the max-tokens notice when the window starts after the owning turn/start', () => {
+    const value = assembler([
+      at(9, 'turn/end', { turn: 3, reason: { kind: 'max-tokens' } }),
+    ], true)
+    const notice = node(snapshot(value), 'turn-max-tokens')
+    expect(notice?.data).toMatchObject({ kind: 'turn-max-tokens', seq: 9, turn: 3 })
+  })
+
+  it('pins the max-tokens Definition edges the engine cannot reach', () => {
+    // The engine only hands start the single matched turn/end and never emits
+    // update Matches for this kind; these direct calls pin the declared
+    // behavior of both required Definition members anyway.
+    const match = (seq: number, type: string, data: unknown) => ({
+      event: { seq, time: seq * 1_000, type, data },
+      view: undefined,
+      role: 'start',
+      location: undefined,
+    }) as unknown as Parameters<typeof turnMaxTokensDefinition.start>[1]
+    const context = (state: unknown, matches: unknown[] = []) => ({
+      key: 'k', kind: 'turn-max-tokens', id: '1', matches, start: undefined, state, current: new Map(),
+    }) as unknown as Parameters<NonNullable<typeof turnMaxTokensDefinition.buildViewNode>>[0]
+    const reader = { previous: () => undefined }
+
+    expect(() => turnMaxTokensDefinition.start(context(undefined), match(1, 'turn/start', { turn: 1 }), reader))
+      .toThrow('turn-max-tokens start requires a max-tokens turn/end')
+    const state = { turn: 1, seq: 5, time: 5_000 }
+    expect(turnMaxTokensDefinition.update(
+      context(state) as Parameters<typeof turnMaxTokensDefinition.update>[0],
+      match(6, 'turn/end', { turn: 1, reason: { kind: 'completed' } }),
+    )).toBe(state)
+    expect(turnMaxTokensDefinition.buildViewNode?.(context(undefined))).toBeNull()
+    expect(turnMaxTokensDefinition.buildViewNode?.(context(
+      undefined,
+      [match(5, 'turn/end', { turn: 1, reason: { kind: 'max-tokens' } })],
+    ))).toMatchObject({ kind: 'turn-max-tokens' })
+  })
+
   it('preserves nested Tools and manual compaction evidence when their start events are outside the window', () => {
     const value = assembler([
       at(12, 'tool/code-dispatch-start', {