gemini: minimal thinking on correlation, token accounting per call

A day of replay runs (server-26#170) spent ~$5 of Gemini on ~7 two-hour
windows (~$0.70 per 290 calls) — several dollars a day per live deployment
for correlation alone — and nothing could say where it went (#45). Gemini
3.x thinks by default and bills it as output; the deprecated
google-generativeai SDK these calls used cannot set a thinking level.

- app/internal/gemini.py: every Gemini call (correlation + transcript
  correction) goes through google-genai with JSON mode, an explicit
  thinking level, and logs in/out/thinking tokens. A model that rejects
  the level is retried without it once and remembered, so the tier is
  never lost to a config param. API failures still raise for ai_health.
- correlator: thinking_level "minimal" (a link/new/orphan choice).
  transcript correction: "low" until a replay shows minimal is safe.
- replay: runs record real Gemini token usage (metrics.gemini_usage),
  shown in the Replay tab.
- requirements: google-genai.

c2-core: 474 pass. Frontend typecheck not run (no Node on this box).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/tests/test_gemini_helper.py

@@ -0,0 +1,70 @@
+"""
+app/internal/gemini.py — thinking level, fallback when a model rejects it,
+and token accounting into a replay's usage sink (server-26#170 cost finding).
+"""
+from types import SimpleNamespace
+from unittest.mock import patch
+
+from app.internal import gemini
+
+
+class _FakeModels:
+    def __init__(self, reject_thinking=False):
+        self.reject_thinking = reject_thinking
+        self.configs = []
+
+    def generate_content(self, model, contents, config):
+        self.configs.append(config)
+        if self.reject_thinking and config.get("thinking_level"):
+            raise RuntimeError("400 INVALID_ARGUMENT: thinking_level is not supported for this model")
+        return SimpleNamespace(
+            text='{"action": "link"}',
+            usage_metadata=SimpleNamespace(prompt_token_count=1200, candidates_token_count=30,
+                                           thoughts_token_count=0),
+        )
+
+
+def _patched(models):
+    client = SimpleNamespace(models=models)
+    return (patch.object(gemini, "_get_client", return_value=client),
+            patch.object(gemini, "_config", lambda level: {"thinking_level": level}))
+
+
+def test_minimal_thinking_by_default_and_usage_lands_in_the_sink():
+    models = _FakeModels()
+    a, b = _patched(models)
+    sink = {}
+    tok = gemini.collect_usage(sink)
+    try:
+        with a, b:
+            assert gemini.generate_json("m1", "p", purpose="correlation") == {"action": "link"}
+    finally:
+        gemini.reset_usage(tok)
+    assert models.configs == [{"thinking_level": "minimal"}]
+    assert sink == {"correlation:m1": {"calls": 1, "in": 1200, "out": 30, "thinking": 0}}
+
+
+def test_model_that_rejects_thinking_level_falls_back_once():
+    models = _FakeModels(reject_thinking=True)
+    a, b = _patched(models)
+    gemini._no_thinking_level.discard("m2")
+    with a, b:
+        gemini.generate_json("m2", "p", purpose="correlation")
+        gemini.generate_json("m2", "p", purpose="correlation")
+    # first call: tried minimal, retried without; second call: straight without
+    assert models.configs == [{"thinking_level": "minimal"}, {"thinking_level": None}, {"thinking_level": None}]
+    gemini._no_thinking_level.discard("m2")
+
+
+def test_other_failures_still_raise_for_ai_health():
+    class Boom(_FakeModels):
+        def generate_content(self, **kw):
+            raise RuntimeError("429 insufficient_quota")
+    a, b = _patched(Boom())
+    with a, b:
+        try:
+            gemini.generate_json("m3", "p", purpose="correlation")
+        except RuntimeError as e:
+            assert "insufficient_quota" in str(e)
+        else:
+            raise AssertionError("should raise")