[Lumea-art]

What

Fixes #610.

The controller's completion proxy answered with a fresh web.Response(status=500)
from its catch-all except even after StreamResponse.prepare() had already put the
status line and headers on the wire. aiohttp then appended a second status line and
headers to the unfinished chunked body. With aiohttp 3.13/3.14, C-parser clients hang
until their own timeout and Python-parser clients report invalid chunk framing.

Changes

controller/frontend.py

• handle_completion() / handle_chat_completion() now share
  _proxy_completion_request(); the duplicated ~55-line bodies are gone and each
  handler keeps its own upstream path and log label.
• _relay_stream_response() commits the response only after prepare() succeeds.
  A failure past that point logs, truncates the connection
  (_abort_downstream_stream()), and returns the already-started response, so no
  second HTTP response is ever written. A failure before prepare() still
  propagates and becomes the ordinary JSON 500, as before.
• result.release() still runs in finally on every path, so upstream connection
  cleanup and router/backend accounting are unchanged.
• Handler return annotations are widened to web.StreamResponse, which is what the
  streaming path actually returns.

Tests

New tests/test_frontend_streaming.py, registered in tests/manifests/cpu.txt
(only needs aiohttp, which the CPU workflow already installs). It runs the real
handlers over a real aiohttp socket and reads the raw response bytes with a
hand-written HTTP request:

• upstream stream fails after headers -> exactly one HTTP/1.1 status line, no
  500, first SSE event still delivered, body left truncated (parametrised over
  both endpoints);
• healthy stream -> chunks relayed and the body terminated with 0\r\n\r\n;
• router raises before headers -> ordinary JSON 500 with the error message;
• missing model -> 400 and the router is never called.

Against the unfixed frontend.py the two truncation tests fail with
assert 2 == data.count(b"HTTP/1.1 "); with the fix all six pass.

Verification

pytest tests/test_frontend_streaming.py tests/test_sleep_manager.py tests/test_traffic_monitor.py
# 31 passed

ruff check controller/frontend.py tests/test_frontend_streaming.py   # All checks passed
isort --check-only controller/frontend.py tests/test_frontend_streaming.py  # clean
mypy --follow-imports skip --namespace-packages tests/test_frontend_streaming.py  # clean
python tools/check_test_classification.py  # test classification valid: total=110

[shipiyouniao]

Verified both completion endpoints through the normal frontend CLI and real router against a controlled HTTP backend (Python 3.11.15, aiohttp 3.14.3).

Base e0e4059 emits a second status line on upstream truncation. This head emits one, preserves the first SSE event, and closes without a final chunk, including keep-alive requests. Healthy streaming, non-streaming JSON, missing-model and routing-error paths also passed.

The new tests fail 2/6 on base and pass 6/6 on head. All nine CI checks are green. No blocking findings.