feat(fase2): fail-fast contract loaders

tests/test_contracts.py

@@ -0,0 +1,50 @@
+"""Step 11 tests — fail-fast contract loaders (malformed config raises at startup, no LLM).
+
+Every malformed contract raises a ValidationError at load — and, critically, no chat-client
+is ever touched (the validation is pure config-layer, ordered before any backend exists).
+Pattern: tests/test_backends.py (raises) + FakeChatClient.call_count spy.
+"""
+
+import pytest
+from pydantic import ValidationError
+from spikes._harness import FakeChatClient
+
+from portfolio_optimiser.contracts import Contracts, load_contracts
+
+_DS = {"docs_dir": "docs", "top_k": 3}
+_TERM = {"max_rounds": 3, "max_tokens": 10000}
+_FB = {"decision": "approved", "rationale": "feasible within range"}
+
+
+def test_valid_contracts_load() -> None:
+    contracts = load_contracts(_DS, _TERM, _FB)
+    assert isinstance(contracts, Contracts)
+    assert contracts.model_map.local["default"]  # bundled model_map validated
+
+
+def test_malformed_data_source_raises() -> None:
+    with pytest.raises(ValidationError):
+        load_contracts({"docs_dir": "docs", "top_k": 0}, _TERM, _FB)  # top_k must be > 0
+
+
+def test_malformed_model_map_raises() -> None:
+    bad_map = {"local": {"proposer": "qwen3:4b"}, "azure": {"default": "x"}}  # local missing 'default'
+    with pytest.raises(ValidationError):
+        load_contracts(_DS, _TERM, _FB, model_map=bad_map)
+
+
+def test_malformed_termination_raises() -> None:
+    with pytest.raises(ValidationError):
+        load_contracts(_DS, {"max_rounds": 0, "max_tokens": 10000}, _FB)  # unbounded
+
+
+def test_malformed_feedback_raises() -> None:
+    with pytest.raises(ValidationError):
+        load_contracts(_DS, _TERM, {"decision": "maybe", "rationale": "x"})  # not a literal
+
+
+def test_no_chat_client_call_on_malformed_contract() -> None:
+    spy = FakeChatClient()
+    with pytest.raises(ValidationError):
+        load_contracts({"docs_dir": "docs", "top_k": -1}, _TERM, _FB)
+    assert spy.call_count == 0  # contract validation never constructs or calls a client