fix(memory): fall back to pip when uv is unavailable (salvage #5954) (#38668)

`_install_dependencies` (hermes memory setup) hard-aborted with
"uv not found — cannot install dependencies" whenever `uv` was not on
PATH, even when a perfectly good `pip` was available. Slim container
images and some CI environments don't ship uv, so memory-provider
dependency installation dead-ended there for no good reason.

Now: use `uv pip install` when uv is present, otherwise fall back to
`<python> -m pip install` when pip3/pip is available, and only abort
(with the uv install hint) when neither is found. The "Run manually:"
hints reflect whichever installer was selected.

Salvages #5954 by @MustafaKara7. Their patch added redundant local
`import subprocess` / `import sys` (both are already in scope — module
-level `sys`, function-top `subprocess`); this salvage drops those and
adds a regression test (TestInstallDependenciesRunner) covering all
three paths (uv / pip-fallback / abort). Verified adversarially: the
pip-fallback test fails against origin/main's unfixed code with the
exact dead-end symptom and passes with the fix.

Closes #5954.

Co-authored-by: MustafaKara7 <186085093+MustafaKara7@users.noreply.github.com>

tests/hermes_cli/test_memory_setup_provider_arg.py

@@ -48,3 +48,50 @@ class TestMemorySetupProviderRouting:
         out = capsys.readouterr().out
         assert "not found" in out
         assert "hermes memory setup" in out
+
+
+class TestInstallDependenciesRunner:
+    """`_install_dependencies` must install via `uv` when present and fall back
+    to standard `pip` when `uv` is unavailable (e.g. slim containers / CI images
+    that don't ship uv) instead of dead-ending with "cannot install"."""
+
+    def _run_with_missing_dep(self, tmp_path, which_side_effect):
+        """Drive _install_dependencies for a plugin that declares one missing
+        pip dep, capturing the subprocess.run argv (or None if never called)."""
+        import sys
+
+        (tmp_path / "plugin.yaml").write_text(
+            "pip_dependencies:\n  - definitely-not-installed-xyz\n", encoding="utf-8"
+        )
+        captured = {}
+
+        def fake_run(cmd, **kw):
+            captured["cmd"] = cmd
+            return SimpleNamespace()
+
+        with patch("plugins.memory.find_provider_dir", return_value=tmp_path), \
+             patch("shutil.which", side_effect=which_side_effect), \
+             patch("subprocess.run", fake_run):
+            memory_setup._install_dependencies("x")
+        return captured.get("cmd"), sys.executable
+
+    def test_uses_uv_when_available(self, tmp_path):
+        cmd, _ = self._run_with_missing_dep(
+            tmp_path, lambda b: "/usr/bin/uv" if b == "uv" else None
+        )
+        assert cmd is not None
+        assert cmd[:3] == ["/usr/bin/uv", "pip", "install"]
+
+    def test_falls_back_to_pip_when_uv_missing(self, tmp_path, capsys):
+        """The salvaged behavior (#5954): no uv but pip present -> python -m pip."""
+        cmd, py = self._run_with_missing_dep(
+            tmp_path, lambda b: "/usr/bin/pip3" if b == "pip3" else None
+        )
+        assert cmd is not None
+        assert cmd[:4] == [py, "-m", "pip", "install"]
+        assert "Falling back to standard pip" in capsys.readouterr().out
+
+    def test_aborts_when_neither_uv_nor_pip(self, tmp_path, capsys):
+        cmd, _ = self._run_with_missing_dep(tmp_path, lambda b: None)
+        assert cmd is None  # no install attempted
+        assert "cannot install dependencies" in capsys.readouterr().out