fix(web): grant turn-tail IconActions only after the turn ends

`assistantActionsSeqs` picked the last content-text assistant of each turn
from the finalized transcript alone. That quantity is stable only once the
turn closes: while a turn is still producing steps, the narration written
before a tool call is the last content assistant so far, so copy, branch,
and the clock appeared under an intermediate sentence for as long as the
tool ran and then moved down to the next step's text.

Pass `ConversationSnapshot.turnEnds` into the derivation and grant the row
only inside a turn that has a durable `turn/end`. This is the same
completion fact the branch control and the `Ran for` label already read, so
the three parts of one row now agree; mid-turn narration owns nothing, and
the seat appears once under the settled answer.

`hasContentText` moves to chat-flow.ts so the ownership gate and
AssistantMarkdown's mount gate cannot drift apart.

apps/web/tests/turn-tail-actions.e2e.ts pins both states through the
assembled application: a hang sidecar on the second model call parks a turn
whose first step narrated before calling bash, and the two goldens hold the
parked flow and the flow after stopping.

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -225,12 +225,12 @@ describe('chat-flow derivation', () => {
     expect(flowKeys(deriveChatFlow([toolResult(3, 'a'), assistant(4, 'found'), toolResult(5, 'b')]))).toBe('g3|n4|g5')
   })
 
-  it('assistantActionsSeqs keeps only the last content assistant per turn', () => {
+  it('assistantActionsSeqs keeps only the last content assistant per completed turn', () => {
     const thinkOnly: AssistantMessageNode = {
       kind: 'assistant', seq: 3, time: 3_000, turn: 1, step: 2,
       blocks: [{ kind: 'reasoning', text: 'planning' }],
     }
-    const seqs = assistantActionsSeqs([
+    const nodes: ConversationNode[] = [
       user(1, 'hi'),
       assistant(2, 'looking', 1),
       thinkOnly,
@@ -238,8 +238,11 @@ describe('chat-flow derivation', () => {
       assistant(5, 'done', 1),
       user(6, 'again'),
       assistant(7, 'second turn', 2),
-    ])
-    expect([...seqs].sort((a, b) => a - b)).toEqual([5, 7])
+    ]
+    expect([...assistantActionsSeqs(nodes, new Map([[1, 5], [2, 7]]))].sort((a, b) => a - b)).toEqual([5, 7])
+    // Turn 2 is still producing steps: its latest narration owns nothing, and
+    // the settled turn 1 keeps its seat.
+    expect([...assistantActionsSeqs(nodes, new Map([[1, 5]]))]).toEqual([5])
   })
 
   it('runningTurnStartTime selects the latest turn/start without a turn/end', () => {
@@ -401,7 +404,9 @@ describe('ChatView', () => {
     expect(view.getAllByText('interrupt now')).toHaveLength(1)
     expect(view.container.querySelector('[data-pending-steering]')).toBeNull()
     expect(view.getAllByText('插话')).toHaveLength(1)
-    expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(2)
+    // Only the durable steering bubble: the turn is still running, so its
+    // assistant narration owns no footer yet.
+    expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(1)
     const durableBubble = view.getByText('interrupt now').closest('[class*="userRow"]') as HTMLElement
     const unavailable = within(durableBubble).getByRole('button', { name: '在新对话中分支' })
     expect(unavailable.getAttribute('aria-disabled')).toBe('true')
@@ -525,6 +530,28 @@ describe('ChatView', () => {
     expect(branchButtons.map(button => button.getAttribute('aria-disabled'))).toEqual(['true', null, 'true', null])
   })
 
+  it('withholds assistant IconActions while the turn is still running', () => {
+    const h = makeHarness({
+      running: true,
+      runningCalls: [runningCall('a')],
+      nodes: [
+        user(1, 'first'),
+        assistant(2, 'previous answer', 1),
+        user(3, 'second'),
+        assistant(4, 'mid-turn text', 2),
+      ],
+      turnEnds: new Map([[1, 2]]),
+    })
+    const view = render(<h.ChatView {...h.props} />)
+    // 2 user + the settled turn-1 tail; turn 2's narration stays chrome-free
+    // while its tool runs, so the footer never appears and then moves.
+    expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(3)
+    expect(view.getByText('mid-turn text')).toBeTruthy()
+    // turn/end lands: the same node becomes the settled answer and takes the seat.
+    act(() => { h.set({ running: false, runningCalls: [], turnEnds: new Map([[1, 2], [2, 5]]) }) })
+    expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(4)
+  })
+
   it('the actions-owning assistant footer shows the turn run time', () => {
     const h = makeHarness({
       nodes: [