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.

gateway/run.py

@@ -13277,25 +13277,24 @@ class GatewayRunner(GatewayKanbanWatchersMixin):
             lines.append(t("gateway.usage.label_total", count=f"{agent.session_total_tokens:,}"))
             lines.append(t("gateway.usage.label_api_calls", count=agent.session_api_calls))
 
-            # Cost estimation
+            # Cost — provider-REPORTED only (OpenRouter usage.cost accumulator
+            # and/or Nous credits-header delta). No estimation: when nothing
+            # was reported the line is omitted entirely, never shown as $0.00.
+            # Subscription-included routes (a billing fact, not a price guess)
+            # still show "included".
             try:
-                from agent.usage_pricing import CanonicalUsage, estimate_usage_cost
-                cost_result = estimate_usage_cost(
-                    agent.model,
-                    CanonicalUsage(
-                        input_tokens=input_tokens,
-                        output_tokens=output_tokens,
-                        cache_read_tokens=cache_read,
-                        cache_write_tokens=cache_write,
-                    ),
-                    provider=getattr(agent, "provider", None),
-                    base_url=getattr(agent, "base_url", None),
-                )
-                if cost_result.amount_usd is not None:
-                    prefix = "~" if cost_result.status == "estimated" else ""
-                    lines.append(t("gateway.usage.label_cost", prefix=prefix, amount=f"{float(cost_result.amount_usd):.4f}"))
-                elif cost_result.status == "included":
-                    lines.append(t("gateway.usage.label_cost_included"))
+                from agent.usage_pricing import real_session_cost_usd, resolve_billing_route
+                real_cost = real_session_cost_usd(agent)
+                if real_cost is not None:
+                    lines.append(t("gateway.usage.label_cost", prefix="", amount=f"{real_cost:.4f}"))
+                else:
+                    route = resolve_billing_route(
+                        agent.model,
+                        provider=getattr(agent, "provider", None),
+                        base_url=getattr(agent, "base_url", None),
+                    )
+                    if route.billing_mode == "subscription_included":
+                        lines.append(t("gateway.usage.label_cost_included"))
             except Exception:
                 pass