fix(cookbook): install llama-cpp-python[server] so llama.cpp serving works (#730) (#1338)

The llama.cpp serve auto-install built a bare `llama-cpp-python` in the Linux
source-build fallback and the Termux path, but the serve command runs
`python3 -m llama_cpp.server`, which needs the `[server]` extra. Because the
"already installed?" guard only checks `import llama_cpp` (a bare install
satisfies it), the missing extra was never added, so serving crashed with
`ModuleNotFoundError: No module named 'starlette_context'` (issue #730).

- Request the `[server]` extra in both the Termux direct install and the Linux
  Python-bindings fallback (the Windows path already used `[server]`).
- Shell-quote the package spec in `_pip_install_fallback_chain` via `shlex.quote`
  so the `[server]` brackets aren't treated as a bash glob; plain names unaffected.

Tests: tests/test_cookbook_helpers.py gains extras-quoting coverage and a
serve-runner regression guard.

routes/cookbook_helpers.py

@@ -194,8 +194,13 @@ def _pip_install_fallback_chain(package: str, *, python_cmd: str = "python3 -m p
     pip output appear in the Cookbook log on failure.
     """
     upgrade_flag = " -U" if upgrade else ""
-    base = _pip_install_attempt(f"{python_cmd} install -q{upgrade_flag} {package}")
-    user = _pip_install_attempt(f"{python_cmd} install --user --break-system-packages -q{upgrade_flag} {package}")
+    # Shell-quote the package spec: an extras spec like ``llama-cpp-python[server]``
+    # contains brackets that bash would treat as a glob, so it must be quoted
+    # before being embedded in the install command. Plain names (e.g.
+    # ``huggingface_hub``) are returned unchanged by ``shlex.quote``.
+    pkg = shlex.quote(package)
+    base = _pip_install_attempt(f"{python_cmd} install -q{upgrade_flag} {pkg}")
+    user = _pip_install_attempt(f"{python_cmd} install --user --break-system-packages -q{upgrade_flag} {pkg}")
     # Derive the python executable for the venv detection check.
     # Must use the same interpreter that pip belongs to; hardcoding
     # python3 breaks when pip lives in a venv that only has "python".