feat(fase2): first-class Pydantic provenance stamp

tests/test_provenance.py

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+"""Step 6 tests — first-class Pydantic provenance stamp.
+
+At least one citation is mandatory (the load-bearing NFR); a valid stamp exposes the
+proposal's provenance fields; the annotation adapter yields MAF citation dicts for display.
+Pattern: tests/spikes/test_c_validator.py (pydantic raises + field assertions).
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from portfolio_optimiser.provenance import Citation, ProvenanceStamp
+from portfolio_optimiser.retrieval import TextSpan
+
+
+def _stamp() -> ProvenanceStamp:
+    return ProvenanceStamp(
+        citations=[Citation(file="asphalt.txt", locator=TextSpan(0, 10), snippet="0123456789")],
+        model="qwen3:4b",
+        role="proposer",
+        validator_decision="validated",
+        token_usage=42,
+    )
+
+
+def test_zero_citations_is_rejected() -> None:
+    with pytest.raises(ValidationError):
+        ProvenanceStamp(
+            citations=[],
+            model="m",
+            role="proposer",
+            validator_decision="validated",
+            token_usage=1,
+        )
+
+
+def test_valid_stamp_exposes_provenance_fields() -> None:
+    stamp = _stamp()
+    assert len(stamp.citations) >= 1
+    assert stamp.model and stamp.role
+    assert stamp.validator_decision in {"validated", "rejected"}
+    assert isinstance(stamp.token_usage, int) and stamp.token_usage > 0
+
+
+def test_to_annotations_yields_citation_dicts() -> None:
+    anns = _stamp().to_annotations()
+    assert anns and all(a["type"] == "citation" for a in anns)
+    assert anns[0]["annotated_regions"][0]["type"] == "text_span"