test(companion): Add config validation tests

Cover validate_companion_workers (None becomes empty, non-list and non-dict
items rejected) and CompanionConfig.config_hash (stable for equal configs,
changes when a field changes, callable target keyed by qualified name and
hashed stably).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/companion-process-manager.md

@@ -695,7 +695,7 @@ No per-companion logic in Arbiter.
 - [x] Close manager-only fds in companion child.
 - [x] Add parent-death cleanup.
 - [x] Add lifecycle logs.
-- [ ] Add tests for config validation.
+- [x] Add tests for config validation.
 - [ ] Add tests for state transitions.
 - [ ] Add tests for control commands.
 - [ ] Add tests for transactional reread.

tests/test_companion_config.py

@@ -4,8 +4,8 @@
 
 import pytest
 
-from gunicorn.config import Config
+from gunicorn.config import Config, validate_companion_workers
-from gunicorn.companion.config import build_companion_configs
+from gunicorn.companion.config import CompanionConfig, build_companion_configs
 
 
 def make_config(workers, **overrides):
@@ -45,3 +45,38 @@ def test_build_requires_name_and_target():
         build_companion_configs(make_config([{"name": "rq"}]))
     with pytest.raises(ValueError):
         build_companion_configs(make_config([{"target": "pkg:run"}]))
+
+
+def test_validate_companion_workers_accepts_none_and_list():
+    assert validate_companion_workers(None) == []
+    workers = [{"name": "rq", "target": "pkg:run"}]
+    assert validate_companion_workers(workers) == workers
+
+
+def test_validate_companion_workers_rejects_non_list():
+    with pytest.raises(TypeError):
+        validate_companion_workers("rq")
+
+
+def test_validate_companion_workers_rejects_non_dict_item():
+    with pytest.raises(TypeError):
+        validate_companion_workers(["rq"])
+
+
+def test_config_hash_stable_and_field_sensitive():
+    base = CompanionConfig(name="rq", target="pkg:run")
+    same = CompanionConfig(name="rq", target="pkg:run")
+    changed = CompanionConfig(name="rq", target="pkg:run", stop_timeout=99)
+    assert base.config_hash == same.config_hash
+    assert base.config_hash != changed.config_hash
+
+
+def test_config_hash_keys_callable_target_by_qualified_name():
+    def run():
+        pass
+
+    keyed = CompanionConfig._target_key(run)
+    assert ":" in keyed and keyed.endswith("run")
+    # A callable target hashes stably across CompanionConfig instances.
+    assert (CompanionConfig(name="rq", target=run).config_hash
+            == CompanionConfig(name="rq", target=run).config_hash)