feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

tests/e2e/test_thinking_scroll.py

@@ -176,10 +176,13 @@ def send_and_wait(page: Page, question: str) -> None:
     page.evaluate("() => document.querySelector('#composer').requestSubmit()")
     expect(page.locator(".msg.user .bubble").last).to_contain_text(question)
     # The mock streams at 0.02s/chunk, so thinking + answer land in a few
-    # seconds — 30s is generous on headless Chromium.
+    # seconds — 30s is generous on headless Chromium. Phase 48: the label
+    # assertion carries the settle wait with an explicit timeout — the
+    # in-flight button is the enabled Stop control (never disabled), so
+    # to_be_enabled no longer blocks until the turn settles.
     expect(page.locator(".msg.brain .bubble").last).not_to_have_text("", timeout=30_000)
     expect(page.locator("#send-btn")).to_be_enabled(timeout=30_000)
-    expect(page.locator("#send-label")).to_have_text("Send")
+    expect(page.locator("#send-label")).to_have_text("Send", timeout=30_000)
 
 
 def submit(page: Page, question: str) -> None:
@@ -787,8 +790,10 @@ def test_thinking_window_follows_across_paragraph_breaks(
     )
 
     # The turn settles; the scratchpad text past the break is intact and
-    # the answer landed.
+    # the answer landed. Phase 48: the label assertion carries the settle
+    # wait (explicit timeout — the in-flight Stop button is never
+    # disabled, so to_be_enabled no longer blocks until the turn ends).
     expect(page.locator("#send-btn")).to_be_enabled(timeout=30_000)
-    expect(page.locator("#send-label")).to_have_text("Send")
+    expect(page.locator("#send-label")).to_have_text("Send", timeout=30_000)
     expect(details.locator(".thinking-text")).to_contain_text("Step 3")
     expect(page.locator(".msg.brain .bubble").last).to_contain_text(MOCK_ANSWER_MARKER)