Implement client cost attribution

tests/test_attribution.py

@@ -0,0 +1,19 @@
+import pytest
+
+from fin_hub.attribution import ClientAttribution, optional_attribution
+
+
+def test_client_attribution_key_is_stable():
+    attribution = ClientAttribution(" acme ", "portal", "prod-01")
+    assert attribution.key == "client:acme|app:portal|instance:prod-01"
+
+
+def test_empty_attribution_is_explicitly_unattributed():
+    assert optional_attribution(None, None, None) is None
+    assert optional_attribution("", "", "") is None
+
+
+@pytest.mark.parametrize("invalid", ["has space", "has/slash", "", "x" * 129])
+def test_client_attribution_rejects_unsafe_parts(invalid: str):
+    with pytest.raises(ValueError):
+        ClientAttribution(invalid, "portal", "prod-01")