gateway: capture real provider-reported cost (openrouter usage accounting)

Cost displays were estimates from a pricing table; on OpenRouter the
status bar never reflected what was actually charged. Now cost is
provider-REPORTED only, end to end:

- OpenRouter requests carry usage:{include:true} (profile + legacy
  transport paths); the response usage.cost field (credits, 1:1 USD)
  is captured per call into agent.session_actual_cost_usd and
  persisted to the sessions DB actual_cost_usd column (NULL-safe:
  unreported calls never touch the stored value).
- Nous keeps its x-nous-credits-* header capture; the header delta
  now surfaces as the session's real cost via real_session_cost_usd.
- Providers that report nothing accumulate NOTHING: cost fields stay
  absent/None (the TUI hides its cost segment), never a fabricated
  $0.00 and never an estimate. _get_usage, gateway /usage and the
  CLI usage page all switched off estimate_usage_cost for display.
- Per-model session accumulator (session_model_usage) records real
  per-call counts and provider-reported cost per model.

tests/providers/test_provider_profiles.py

@@ -110,7 +110,9 @@ class TestOpenRouterProfile:
     def test_extra_body_no_prefs(self):
         p = get_provider_profile("openrouter")
         body = p.build_extra_body()
-        assert body == {}
+        # Usage accounting is always requested (real provider-reported cost);
+        # nothing else should appear without prefs/session.
+        assert body == {"usage": {"include": True}}
 
     def test_pareto_min_coding_score_emitted_for_pareto_model(self):
         """min_coding_score → plugins block when model is openrouter/pareto-code."""