feat(token-tracking): surface token_usage in ReportData

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

tests/test_report_builder_token_usage.py

@@ -0,0 +1,55 @@
+"""Tests for token_usage passthrough in the webapp report builder."""
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+from webapp.services.report_builder import build_report
+
+
+def _write_minimal_run(run_dir: Path, token_usage: dict | None) -> None:
+    run_dir.mkdir(parents=True, exist_ok=True)
+    (run_dir / "scores.csv").write_text(
+        "sample_id,faithfulness\ns1,0.9\n", encoding="utf-8"
+    )
+    (run_dir / "summary.md").write_text("summary", encoding="utf-8")
+    metadata = {"run_id": run_dir.name}
+    if token_usage is not None:
+        metadata["token_usage"] = token_usage
+    (run_dir / "metadata.json").write_text(json.dumps(metadata), encoding="utf-8")
+
+
+def test_build_report_passes_through_token_usage(tmp_path: Path) -> None:
+    run_dir = tmp_path / "run"
+    _write_minimal_run(
+        run_dir,
+        token_usage={"gpt-5": {"input_tokens": 100, "output_tokens": 50, "calls": 2}},
+    )
+
+    report = build_report(run_dir, ["faithfulness"])
+
+    assert report.token_usage == {
+        "gpt-5": {"input_tokens": 100, "output_tokens": 50, "calls": 2}
+    }
+
+
+def test_build_report_defaults_token_usage_to_empty_dict(tmp_path: Path) -> None:
+    run_dir = tmp_path / "run"
+    _write_minimal_run(run_dir, token_usage=None)
+
+    report = build_report(run_dir, ["faithfulness"])
+
+    assert report.token_usage == {}
+
+
+def test_build_report_early_return_branch_still_surfaces_token_usage(tmp_path: Path) -> None:
+    """metrics=[] forces the early-return branch; token_usage must still surface."""
+    run_dir = tmp_path / "run"
+    _write_minimal_run(
+        run_dir,
+        token_usage={"gpt-5": {"input_tokens": 5, "output_tokens": 5, "calls": 1}},
+    )
+
+    report = build_report(run_dir, [])
+
+    assert report.token_usage == {"gpt-5": {"input_tokens": 5, "output_tokens": 5, "calls": 1}}