feat: record reranker call outcome into ModelUsageTracker

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/observability/test_reranker_usage_tracking.py

@@ -0,0 +1,50 @@
+"""Verifies OpenAICompatibleReranker records call outcome (no tokens) into ModelUsageTracker."""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+
+from app.domain.retrieval import RetrievedChunk
+from app.infrastructure.vectorstore.cross_encoder_reranker import OpenAICompatibleReranker
+from app.shared.model_usage_tracker import get_model_usage_tracker
+
+
+@pytest.fixture(autouse=True)
+def _reset_tracker():
+    """Clear the process-wide tracker before and after each test in this file."""
+    get_model_usage_tracker()._entries.clear()
+    yield
+    get_model_usage_tracker()._entries.clear()
+
+
+def _chunk(chunk_id: str, text: str) -> RetrievedChunk:
+    """Build a minimal RetrievedChunk for reranker tests."""
+    return RetrievedChunk(chunk_id=chunk_id, doc_id="doc-1", doc_title="Doc", text=text, score=0.0)
+
+
+def test_successful_rerank_records_call_without_tokens():
+    """A successful rerank() call is recorded with call_count_ok but zero tokens."""
+    reranker = OpenAICompatibleReranker(base_url="http://example.test", model="bge-reranker-v2-m3")
+    with patch.object(reranker, "_call_reranker", return_value=[0.9, 0.1]):
+        result = reranker.rerank("query", [_chunk("c1", "a"), _chunk("c2", "b")], top_k=2)
+
+    assert len(result) == 2
+    entry = get_model_usage_tracker().get("reranker", "bge-reranker-v2-m3")
+    assert entry is not None
+    assert entry.call_count_ok == 1
+    assert entry.total_tokens == 0
+
+
+def test_failed_rerank_records_error_and_falls_back():
+    """A rerank() call that raises internally is recorded as an error but still returns a fallback list."""
+    reranker = OpenAICompatibleReranker(base_url="http://example.test", model="bge-reranker-v2-m3")
+    with patch.object(reranker, "_call_reranker", side_effect=RuntimeError("gateway down")):
+        result = reranker.rerank("query", [_chunk("c1", "a")], top_k=1)
+
+    assert len(result) == 1  # existing fallback behavior: original order, unscored
+    entry = get_model_usage_tracker().get("reranker", "bge-reranker-v2-m3")
+    assert entry is not None
+    assert entry.call_count_error == 1
+    assert entry.status == "error"