fix(interrupt): keep partial streamed reply when stopped mid-response

[OutThisLife]

Summary

Pressing stop/esc to redirect mid-response made the agent "forget" what it
was saying. When a turn is interrupted while the model is streaming,
InterruptedError was caught, final_response was set to the throwaway
"Operation interrupted: waiting for model response (Xs elapsed)." sentinel, and
messages were persisted without the assistant text already streamed to the
screen. The next turn had no record of the half-finished reply, so the model
restarted blind — reported as "if I stop a message it forgets about it."

The fix recovers the on-screen text from agent._current_streamed_assistant_text
in the InterruptedError branch (agent/conversation_loop.py) and appends it as
the assistant turn, also surfacing it as final_response. The metadata sentinel
is kept only when nothing was streamed yet, so the ACP/client suppression
behavior (INTERRUPT_WAITING_FOR_MODEL_PREFIX startswith check) is unchanged.

Root cause / history

• Lossy from day one — the streaming except InterruptedError handler was
  introduced in c98ee9852
  ("feat: implement interactive prompts for sudo password and command approval",
  Feb 21 2026) with final_response = "Operation interrupted." and no capture of
  the streamed text.
• The fix that missed this path — 397eae5d9
  ("fix: recover partial streamed content on connection failure", Apr 13 2026)
  added the _current_streamed_assistant_text salvage, but only for the
  stream-failure twin — the user-interrupt branch right above it was never
  wired up. This PR completes that work.
• No-op for this bug — 2394e1872
  (sentinel wording) and the refactors 053025238
  / f5bd09af4
  only moved/renamed the handler.

Test plan

• test_interrupt_during_stream_preserves_partial_assistant_text — partial
  reply is appended to history + returned as final_response.
• test_interrupt_before_any_stream_keeps_sentinel — no streamed text falls
  back to the metadata sentinel, tail stays the user turn.
• Existing interrupt/streaming/alternation suites green (streaming,
  partial_stream_finish_reason, turn_finalizer_interrupt_alternation,
  interrupt_propagation, concurrent_interrupt — 60 passed).

[github-actions]

🔎 Lint report: bb/fix-interrupt-partial-reply vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 11396 on HEAD, 11396 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 6001 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.