fix(engines): pin uv pip install to the running interpreter (#529/#527) (#559)

The engine Install chip (deep_translator / openai / argostranslate) 500'd with
"No virtual environment found". run_pip shells out to bare `uv pip install`,
which discovers its target venv from VIRTUAL_ENV / a .venv in CWD — NOT from the
running interpreter. The desktop spawns `<venv>/bin/python -m uvicorn` without
exporting VIRTUAL_ENV and CWDs outside the venv, so uv finds nothing. The
existing `--system` fallback never fires because it's gated on _in_virtualenv()
being False, but the running interpreter genuinely IS in a venv (it just can't be
auto-discovered) — the heuristic answers the wrong question.

Pass `--python sys.executable` for uv install/uninstall: targets the same
interpreter _probe()/is_installed() import from, fixing the whole class
(spawned-venv, system, conda). It takes precedence when both flags are present,
so the Docker `--system` path is untouched. (#527's openai is already bundled by
#484; this hardens the chip for the remaining runtime-installed engines.)

Test: run_pip's spawned argv contains `--python <sys.executable>` after
install/uninstall. 2 passed.

Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/services/translation_engines.py

@@ -177,6 +177,16 @@ async def run_pip(args: list[str], timeout: float = 600.0) -> tuple[int, str]:
     """
     base = _installer_cmd()
     using_uv = base[:1] == ["uv"]
+    # Pin `uv pip` to the interpreter the backend ACTUALLY runs under. The desktop
+    # spawns `<venv>/bin/python -m uvicorn` WITHOUT exporting VIRTUAL_ENV, so bare
+    # `uv pip install` finds no venv and 500s with "No virtual environment found"
+    # (#529/#527) — and the `--system` branch below never fires, because the
+    # running interpreter genuinely IS in a venv (uv just can't auto-discover it).
+    # `--python sys.executable` targets the same interpreter _probe()/is_installed()
+    # import from, and takes precedence when both flags are present, so the Docker
+    # `--system` path is unaffected.
+    if using_uv and args and args[0] in ("install", "uninstall") and "--python" not in args:
+        args = [args[0], "--python", sys.executable, *args[1:]]
     if using_uv and not _in_virtualenv() and args and args[0] in ("install", "uninstall") and "--system" not in args:
         args = [args[0], "--system", *args[1:]]
     cmd = base + args

tests/test_translation_engine_install.py

@@ -0,0 +1,45 @@
+"""The engine Install chip must target the interpreter the backend runs under.
+
+#529/#527: the desktop spawns `<venv>/bin/python -m uvicorn` WITHOUT exporting
+VIRTUAL_ENV, so bare `uv pip install` finds no venv and 500s with "No virtual
+environment found". run_pip must pass `--python sys.executable`.
+"""
+import asyncio
+import sys
+
+from services import translation_engines as te
+
+
+class _FakeProc:
+    returncode = 0
+
+    async def communicate(self):
+        return (b"ok", b"")
+
+
+def _run_capturing(monkeypatch, args):
+    """Force the uv branch + capture the spawned argv; return (rc, argv)."""
+    captured = {}
+    monkeypatch.setattr(te.shutil, "which", lambda name: "/usr/bin/uv" if name == "uv" else None)
+
+    async def fake_exec(*argv, **kwargs):
+        captured["argv"] = list(argv)
+        return _FakeProc()
+
+    monkeypatch.setattr(te.asyncio, "create_subprocess_exec", fake_exec)
+    rc, _out = asyncio.run(te.run_pip(args))
+    return rc, captured.get("argv", [])
+
+
+def test_run_pip_pins_uv_install_to_sys_executable(monkeypatch):
+    rc, argv = _run_capturing(monkeypatch, ["install", "deep_translator"])
+    assert rc == 0
+    assert argv[:3] == ["uv", "pip", "install"], argv
+    assert "--python" in argv, argv
+    assert argv[argv.index("--python") + 1] == sys.executable
+
+
+def test_run_pip_pins_uv_uninstall_to_sys_executable(monkeypatch):
+    _rc, argv = _run_capturing(monkeypatch, ["uninstall", "deep_translator"])
+    assert "--python" in argv, argv
+    assert argv[argv.index("--python") + 1] == sys.executable