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/cli/test_cli_status_bar.py

@@ -523,7 +523,7 @@ class TestCLIStatusBar:
 
 
 class TestCLIUsageReport:
-    def test_show_usage_includes_estimated_cost(self, capsys):
+    def test_show_usage_reports_real_provider_cost(self, capsys):
         cli_obj = _attach_agent(
             _make_cli(),
             prompt_tokens=10_230,
@@ -535,20 +535,22 @@ class TestCLIUsageReport:
             compressions=1,
         )
         cli_obj.verbose = False
+        # Provider-reported cost (e.g. OpenRouter usage accounting accumulator).
+        cli_obj.agent.session_actual_cost_usd = 0.0640
 
         cli_obj._show_usage()
         output = capsys.readouterr().out
 
         assert "Model:" in output
-        assert "Cost status:" in output
-        assert "Cost source:" in output
-        assert "Total cost:" in output
+        assert "Cost (provider-reported):" in output
         assert "$" in output
         assert "0.064" in output
         assert "Session duration:" in output
         assert "Compressions:" in output
 
-    def test_show_usage_marks_unknown_pricing(self, capsys):
+    def test_show_usage_unreported_cost_is_not_a_dollar_figure(self, capsys):
+        """No estimation: when the provider reports nothing, /usage must NOT
+        fabricate a dollar amount — not even $0.00."""
         cli_obj = _attach_agent(
             _make_cli(model="local/my-custom-model"),
             prompt_tokens=1_000,
@@ -563,13 +565,15 @@ class TestCLIUsageReport:
         cli_obj._show_usage()
         output = capsys.readouterr().out
 
-        assert "Total cost:" in output
-        assert "n/a" in output
-        assert "Pricing unknown for local/my-custom-model" in output
+        assert "not reported by provider" in output
+        assert "Cost (provider-reported):" not in output
+        assert "$0.00" not in output
 
-    def test_zero_priced_provider_models_stay_unknown(self, capsys):
+    def test_show_usage_never_estimates_even_with_known_pricing(self, capsys):
+        """A model with a pricing-table entry must still show NO cost when the
+        provider reported nothing (hard requirement: real cost only)."""
         cli_obj = _attach_agent(
-            _make_cli(model="glm-5"),
+            _make_cli(model="anthropic/claude-sonnet-4-6"),
             prompt_tokens=1_000,
             completion_tokens=500,
             total_tokens=1_500,
@@ -582,9 +586,8 @@ class TestCLIUsageReport:
         cli_obj._show_usage()
         output = capsys.readouterr().out
 
-        assert "Total cost:" in output
-        assert "n/a" in output
-        assert "Pricing unknown for glm-5" in output
+        assert "not reported by provider" in output
+        assert "Cost (provider-reported):" not in output
 
 
 class TestStatusBarWidthSource: