test(vendor_capabilities): red phase for registry lookup, fallback, unknown vendor

3 failing tests in tests/test_vendor_capabilities.py that establish the
core behaviors of the new VendorCapability matrix:

1. test_registry_lookup_known_model: registering and looking up a specific
   (vendor, model) entry returns the registered entry
2. test_fallback_to_vendor_default: looking up an unregistered model returns
   the vendor's '*' default entry
3. test_unknown_vendor_raises: looking up a vendor with no entries raises
   KeyError with a 'No capabilities registered' message

All 3 tests fail with ModuleNotFoundError: No module named
'src.vendor_capabilities' (confirmed via pytest). The implementation file
will be created in the next commit (Green phase).

The autouse _clean_registry fixture snapshots src.vendor_capabilities._REGISTRY
before each test and restores it after, providing test isolation for the
module-level state.

tests/test_vendor_capabilities.py

@@ -0,0 +1,40 @@
+import pytest
+from src.vendor_capabilities import VendorCapabilities, get_capabilities, register
+
+@pytest.fixture(autouse=True)
+def _clean_registry():
+ import src.vendor_capabilities
+ snapshot = src.vendor_capabilities._REGISTRY.copy()
+ yield
+ src.vendor_capabilities._REGISTRY.clear()
+ src.vendor_capabilities._REGISTRY.update(snapshot)
+
+def test_registry_lookup_known_model():
+ caps = VendorCapabilities(
+  vendor='qwen',
+  model='qwen-max',
+  vision=False,
+  context_window=32768
+ )
+ register(caps)
+ retrieved = get_capabilities('qwen', 'qwen-max')
+ assert retrieved.vendor == 'qwen'
+ assert retrieved.model == 'qwen-max'
+ assert retrieved.context_window == 32768
+ assert retrieved.vision is False
+
+def test_fallback_to_vendor_default():
+ caps = VendorCapabilities(
+  vendor='llama',
+  model='*',
+  context_window=131072,
+  cost_tracking=False
+ )
+ register(caps)
+ retrieved = get_capabilities('llama', 'llama-3.3-70b-specdec')
+ assert retrieved.context_window == 131072
+ assert retrieved.cost_tracking is False
+
+def test_unknown_vendor_raises():
+ with pytest.raises(KeyError, match='No capabilities registered'):
+  get_capabilities('nonexistent_vendor', 'anymodel')