ci: convert working-directory to validated dropdown (#36575)

Convert the `working-directory` input in the release workflow from a
free-text string to a dropdown of known package paths.

## Changes
- Change `working-directory` from `type: string` to `type: choice` in
`_release.yml`, enumerating all 21 releasable packages under `libs/` and
`libs/partners/`
- Add `check-release-options` CI job in `check_diffs.yml` that runs a
pytest script to assert the dropdown options match directories
containing a `pyproject.toml`

.github/scripts/test_release_options.py

@@ -0,0 +1,48 @@
+"""Verify _release.yml dropdown options match actual package directories."""
+
+from pathlib import Path
+
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+
+
+def _get_release_options() -> list[str]:
+    workflow = REPO_ROOT / ".github" / "workflows" / "_release.yml"
+    with open(workflow) as f:
+        data = yaml.safe_load(f)
+    try:
+        # PyYAML (YAML 1.1) parses the bare key `on` as boolean True
+        return data[True]["workflow_dispatch"]["inputs"]["working-directory"]["options"]
+    except (KeyError, TypeError) as e:
+        msg = f"Could not find workflow_dispatch options in {workflow}: {e}"
+        raise AssertionError(msg) from e
+
+
+def _get_package_dirs() -> set[str]:
+    libs = REPO_ROOT / "libs"
+    dirs: set[str] = set()
+    # Top-level packages (libs/core, libs/langchain, etc.)
+    for p in libs.iterdir():
+        if p.is_dir() and (p / "pyproject.toml").exists():
+            dirs.add(f"libs/{p.name}")
+    # Partner packages (libs/partners/*)
+    partners = libs / "partners"
+    if partners.exists():
+        for p in partners.iterdir():
+            if p.is_dir() and (p / "pyproject.toml").exists():
+                dirs.add(f"libs/partners/{p.name}")
+    return dirs
+
+
+def test_release_options_match_packages() -> None:
+    options = set(_get_release_options())
+    packages = _get_package_dirs()
+    missing_from_dropdown = packages - options
+    extra_in_dropdown = options - packages
+    assert not missing_from_dropdown, (
+        f"Packages on disk missing from _release.yml dropdown: {missing_from_dropdown}"
+    )
+    assert not extra_in_dropdown, (
+        f"Dropdown options with no matching package directory: {extra_in_dropdown}"
+    )