feat(evidence): define source and corpus contracts

harness/tests/test_corpus_models.py

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+from datetime import UTC, datetime
+
+import pytest
+from pydantic import ValidationError
+
+from tht.corpus.models import CanonicalChunk, CanonicalDocument, CorpusManifest
+
+
+def document(source_uri: str = "https://host/a.md") -> CanonicalDocument:
+    return CanonicalDocument(
+        document_id="doc:abc",
+        source_id="source:a",
+        source_uri=source_uri,
+        source_fingerprint="etag:abc",
+        content_hash="sha256:def",
+        title="A",
+        content="# A",
+        media_type="text/markdown",
+        pipeline_version="normalize-v1",
+    )
+
+
+def chunk() -> CanonicalChunk:
+    return CanonicalChunk(
+        chunk_id="chunk:abc:0",
+        document_id="doc:abc",
+        ordinal=0,
+        content="# A",
+        content_hash="sha256:def",
+        source_uri="https://host/a.md",
+        pipeline_version="chunk-v1",
+    )
+
+
+def test_manifest_contains_provenance_without_credentials():
+    manifest = CorpusManifest(
+        manifest_id="manifest:abc",
+        created_at=datetime(2026, 7, 12, tzinfo=UTC),
+        pipeline_version="evidence-v1",
+        embedding_model="nomic-embed-text",
+        embedding_dimensions=768,
+        documents=[document()],
+        chunks=[chunk()],
+    )
+
+    payload = manifest.model_dump_json()
+
+    assert "https://host/a.md" in payload
+    assert "etag:abc" in payload
+    assert "evidence-v1" in payload
+    assert "nomic-embed-text" in payload
+    assert "api_key" not in payload
+
+
+def test_manifest_can_be_assembled_before_publish_identifiers_are_assigned():
+    manifest = CorpusManifest(documents=[document()])
+
+    assert manifest.documents[0].source_uri == "https://host/a.md"
+    assert manifest.manifest_id is None
+
+
+@pytest.mark.parametrize("model", [document(), chunk()])
+def test_canonical_records_are_frozen(model):
+    with pytest.raises(ValidationError):
+        model.pipeline_version = "changed"  # type: ignore[misc]
+
+
+def test_manifest_collections_have_independent_defaults():
+    first = CorpusManifest(
+        manifest_id="one", created_at=datetime.now(UTC), pipeline_version="v1"
+    )
+    second = CorpusManifest(
+        manifest_id="two", created_at=datetime.now(UTC), pipeline_version="v1"
+    )
+
+    first.documents.append(document())
+
+    assert second.documents == []
+
+
+def test_manifest_validates_embedding_compatibility_fields():
+    with pytest.raises(ValidationError):
+        CorpusManifest(
+            manifest_id="bad",
+            created_at=datetime.now(UTC),
+            pipeline_version="v1",
+            embedding_dimensions=0,
+        )
+
+
+def test_canonical_metadata_rejects_secrets_and_non_json_values():
+    with pytest.raises(ValidationError, match="credential-like"):
+        CanonicalDocument.model_validate(
+            {**document().model_dump(), "metadata": {"password": "secret"}}
+        )
+    with pytest.raises(ValidationError):
+        CanonicalChunk(
+            chunk_id="c",
+            document_id="d",
+            ordinal=0,
+            content="x",
+            content_hash="sha256:x",
+            source_uri="file:///x",
+            pipeline_version="v1",
+            metadata={"bad": object()},
+        )