feat: cloud adapters E2B/Modal and billing export (SAND-WP-0010)

Add credentialed E2B and Modal extensions, burst routing fallback,
fin-hub meter export hook, BYOK docs, and 77 tests.

tests/test_billing_export.py

@@ -0,0 +1,50 @@
+"""fin-hub billing export tests."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+from unittest.mock import MagicMock, patch
+
+from sandboxer.models import ActorType, Consumer, MeterRecord, SandboxState, SandboxStatus
+from sandboxer.payments.billing_export import export_meter_usage
+
+
+def test_export_skipped_when_url_unset() -> None:
+    now = datetime.now(UTC)
+    status = SandboxStatus(
+        sandbox_id="s1",
+        profile_id="profile.e2b-burst",
+        extension_id="ext.e2b",
+        state=SandboxState.DESTROYED,
+        consumer=Consumer(actor=ActorType.ADM, project="sand-boxer"),
+        created_at=now,
+        updated_at=now,
+    )
+    meter = MeterRecord(pricing_model="metered", actual_usd=0.5, duration_s=100.0)
+    assert export_meter_usage(status, extension_id="ext.e2b", meter=meter) is None
+
+
+def test_export_posts_when_configured(monkeypatch) -> None:
+    monkeypatch.setenv("SANDBOXER_FIN_HUB_URL", "http://fin-hub.test")
+    now = datetime.now(UTC)
+    status = SandboxStatus(
+        sandbox_id="s1",
+        profile_id="profile.e2b-burst",
+        extension_id="ext.e2b",
+        state=SandboxState.DESTROYED,
+        consumer=Consumer(actor=ActorType.ADM, project="sand-boxer"),
+        created_at=now,
+        updated_at=now,
+    )
+    meter = MeterRecord(pricing_model="metered", actual_usd=0.5, duration_s=100.0)
+
+    mock_response = MagicMock()
+    mock_response.json.return_value = {"ok": True}
+    with patch("sandboxer.payments.billing_export.httpx.post", return_value=mock_response) as post:
+        result = export_meter_usage(status, extension_id="ext.e2b", meter=meter)
+
+    assert result == {"ok": True}
+    post.assert_called_once()
+    payload = post.call_args.kwargs["json"]
+    assert payload["sandbox_id"] == "s1"
+    assert payload["actual_usd"] == 0.5