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_loading_feedback.py

@@ -9,7 +9,8 @@ Determinism comes from two mock-LLM behaviors (tests/e2e/mock_llm.py):
 
 * ``pretend to think slowly`` in the user message → a 3s warm-up before
   the first token, wide enough to assert the pre-token UI (typing dots +
-  busy "Thinking…" button) at a known timestamp;
+  the enabled Stop button — phase 48 revised the busy-button contract) at
+  a known timestamp;
 * the ``POST /__shutdown__`` hook → the ``llm_down`` fixture stops the
   shared mock to simulate an LLM outage, then restores a fresh instance
   on the same port so later tests keep working.
@@ -205,8 +206,10 @@ def test_typing_indicator_during_slow_think(
 def test_button_state_machine(
     page: Page, app_url: str, mock_llm: int, db_ready: None
 ) -> None:
-    """AC1/AC3: disabled + spinner + 'Thinking…' while in flight; enabled
-    + 'Send' + focused input after done."""
+    """AC1/AC3 (phase 48 revised contract, owner-locked 2026-08-29):
+    in flight the button is the enabled Stop control — "Stop" label,
+    .is-stop class, spinner hidden; after done it is enabled + "Send"
+    (class removed) + the input is focused back."""
     _reset_db(mock_llm, seed=True)
     page.set_default_timeout(30_000)
     page.goto(app_url)
@@ -216,15 +219,17 @@ def test_button_state_machine(
 
     btn = page.locator("#send-btn")
     label = page.locator("#send-label")
-    expect(btn).to_be_disabled(timeout=500)
-    expect(label).to_have_text("Thinking…")
-    expect(btn.locator(".spinner")).to_be_visible()
+    expect(btn).to_be_enabled(timeout=500)
+    expect(label).to_have_text("Stop")
+    expect(btn).to_have_class(re.compile(r"is-stop"))
+    expect(btn.locator(".spinner")).to_be_hidden()
     expect(page.locator("#send-status")).to_contain_text("thinking")
 
-    # Done: button recovers and the input is focused back.
+    # Done: button recovers to the Send state and the input is focused back.
     page.locator(ANSWER).wait_for(state="visible", timeout=30_000)
     expect(label).to_have_text("Send", timeout=30_000)
     expect(btn).to_be_enabled()
+    expect(btn).not_to_have_class(re.compile(r"is-stop"))
     expect(btn.locator(".spinner")).to_be_hidden()
     expect(page.locator("#message-input")).to_be_focused()
 
@@ -266,8 +271,10 @@ def test_streaming_appends_live(
 def test_reduced_motion_keeps_feedback(
     page: Page, app_url: str, mock_llm: int, db_ready: None
 ) -> None:
-    """AC7: under prefers-reduced-motion the dots/spinner stay visible
-    (static/slower) — feedback is never removed, only calmed."""
+    """AC7: under prefers-reduced-motion the typing dots stay visible
+    (static) — feedback is never removed, only calmed. Phase 48: the
+    in-flight button is the enabled Stop control (the spinner is no
+    longer part of the in-flight feedback)."""
     _reset_db(mock_llm, seed=True)
     page.emulate_media(reduced_motion="reduce")
     page.set_default_timeout(30_000)
@@ -278,7 +285,9 @@ def test_reduced_motion_keeps_feedback(
 
     expect(page.locator(TYPING)).to_be_visible(timeout=500)
     expect(page.locator(TYPING).locator(".bubble span")).to_have_count(3)
-    expect(page.locator("#send-btn .spinner")).to_be_visible()
+    expect(page.locator("#send-btn")).to_have_class(re.compile(r"is-stop"))
+    expect(page.locator("#send-label")).to_have_text("Stop")
+    expect(page.locator("#send-btn .spinner")).to_be_hidden()
 
     # And the turn still completes and recovers.
     expect(page.locator("#send-label")).to_have_text("Send", timeout=30_000)