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_sources_midstream_bug.py

@@ -188,8 +188,10 @@ def test_partial_answer_survives_sources_nav_midstream(
     bubble = page.locator(".msg.brain .bubble").last
     expect(bubble).to_contain_text(FIRST_LINE_DOM, timeout=30_000)
     # The turn is still in flight (the stream runs ~9s; navigation takes
-    # well under that).
-    expect(page.locator("#send-btn")).to_be_disabled()
+    # well under that) — phase 48: the in-flight button is the enabled
+    # Stop control, not the old disabled busy state.
+    expect(page.locator("#send-btn")).to_be_enabled()
+    expect(page.locator("#send-label")).to_have_text("Stop")
 
     # THE BUG REPORT, VERBATIM: click "Sources" while chat is generating.
     page.click("#nav-sources")
@@ -257,9 +259,10 @@ def test_no_orphan_brain_message_when_navigated_before_first_token(
     thinking = page.locator(".msg.brain").last.locator("details.thinking")
     thinking.wait_for(state="attached", timeout=10_000)
     expect(thinking.locator(".thinking-text")).to_contain_text(THINKING_TAIL)
-    # Still pre-token: the button is busy with the Thinking state.
-    expect(page.locator("#send-btn")).to_be_disabled()
-    expect(page.locator("#send-label")).to_have_text("Thinking…")
+    # Still pre-token: the button is the enabled Stop control (phase 48 —
+    # the old disabled "Thinking…" busy state is gone).
+    expect(page.locator("#send-btn")).to_be_enabled()
+    expect(page.locator("#send-label")).to_have_text("Stop")
 
     # Leave during the pause (no answer token has streamed — acc is empty,
     # so the pagehide save point must persist nothing brain-side).