chore(toxgen): Check availability of pip and add detail to exceptions (#5076)

Exit `populate_tox.py` early if `pip` module is not available, and add a
suggestion for running the script.

Author: alexander-alderman-webb

scripts/populate_tox/README.md

@@ -7,6 +7,20 @@ sure we support everything we claim to.
 This `populate_tox.py` script is responsible for picking reasonable versions to
 test automatically and generating parts of `tox.ini` to capture this.
 
+## Running the script
+
+You require a free-threaded interpreter with pip installed to run the script. With
+a recent version of `uv` you can directly run the script with the following
+command:
+
+```
+uv run --python 3.14t \
+  --with pip \
+  --with-requirements scripts/populate_tox/requirements.txt \
+  --with-editable . \
+  python scripts/populate_tox/populate_tox.py
+```
+
 ## How it works
 
 There is a template in this directory called `tox.jinja` which contains a

scripts/populate_tox/populate_tox.py

@@ -891,7 +891,31 @@ def _normalize_package_dependencies(package_dependencies: list[dict]) -> list[di
 
 def _exit_if_not_free_threaded_interpreter():
     if "free-threading build" not in sys.version:
-        raise Exception("Running with a free-threaded interpreter is required.")
+        exc = Exception("Running with a free-threaded interpreter is required.")
+        exc.add_note(
+            "A dry run of pip is used to determine free-threading support of packages."
+        )
+        raise exc
+
+
+def _exit_if_pip_unavailable():
+    pip_help_return_code = subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "pip",
+            "--help",
+        ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+    ).returncode
+
+    if pip_help_return_code != 0:
+        exc = Exception("pip must be available.")
+        exc.add_note(
+            "A dry run of pip is used to determine free-threading support of packages."
+        )
+        raise exc
 
 
 def main() -> dict[str, list]:
@@ -901,6 +925,7 @@ def main() -> dict[str, list]:
     global MIN_PYTHON_VERSION, MAX_PYTHON_VERSION
 
     _exit_if_not_free_threaded_interpreter()
+    _exit_if_pip_unavailable()
 
     meta = _fetch_sdk_metadata()
     sdk_python_versions = _parse_python_versions_from_classifiers(