fix(cookbook): stop Ollama runner from executing the install one-liner (#3926)

The generated bash runner printed the missing-ollama hint with the install
one-liner wrapped in backticks inside a double-quoted echo. Backticks in
double quotes are command substitution, so on any serve target without
ollama the script downloaded and ran the system-wide installer (including
remote SSH hosts) instead of printing the hint. _validate_serve_cmd rejects
backticks in user-supplied commands for exactly this reason; the app's own
generated script never goes through that validator.

Move the hint into OLLAMA_MISSING_HINT in cookbook_helpers (no substitution
tokens) and emit it single-quoted via _bash_squote. Tests assert the hint
has no expansion tokens, that no generated echo line carries backticks
inside double quotes, and that bash prints the line literally.

Fixes #3816

tests/test_ollama_runner_hint.py

@@ -0,0 +1,57 @@
+"""The generated Ollama runner must print the install hint, not execute it.
+
+The runner script emitted by /api/model/serve contained:
+
+    echo "ERROR: Ollama not found ... or `curl -fsSL .../install.sh | sh`."
+
+Backticks inside double quotes are bash command substitution, so on any host
+without ollama the script downloaded and ran the system-wide installer
+(including remote SSH serve targets) instead of printing the hint. The hint
+now lives in OLLAMA_MISSING_HINT, contains no substitution tokens, and is
+emitted single-quoted.
+"""
+import os
+import shutil
+import subprocess
+
+import pytest
+
+from routes.cookbook_helpers import OLLAMA_MISSING_HINT, _bash_squote
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+
+
+def test_hint_has_no_shell_expansion_tokens():
+    assert "`" not in OLLAMA_MISSING_HINT
+    assert "$(" not in OLLAMA_MISSING_HINT
+
+
+def test_hint_still_tells_the_user_how_to_install():
+    assert "https://ollama.com/download" in OLLAMA_MISSING_HINT
+    assert "install.sh" in OLLAMA_MISSING_HINT
+
+
+def test_no_runner_echo_line_uses_backticks_in_double_quotes():
+    # Source-level guard: generated-script echo lines must never carry
+    # backticks inside a double-quoted bash string again.
+    src = open(os.path.join(ROOT, "routes", "cookbook_routes.py"), encoding="utf-8").read()
+    offenders = [
+        line.strip()
+        for line in src.splitlines()
+        if "append(" in line and 'echo "' in line and "`" in line.split('echo "', 1)[1]
+    ]
+    assert offenders == []
+
+
+def test_single_quoted_echo_prints_hint_literally():
+    bash = shutil.which("bash")
+    if not bash:
+        pytest.skip("bash not available")
+    out = subprocess.run(
+        [bash, "-c", f"echo '{_bash_squote(OLLAMA_MISSING_HINT)}'"],
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+    assert out.returncode == 0
+    assert out.stdout.strip() == OLLAMA_MISSING_HINT