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_modal.py

@@ -0,0 +1,63 @@
+"""Modal cloud adapter tests."""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from sandboxer.extensions.modal import ModalExtension
+from sandboxer.models import Profile
+
+
+def _profile() -> Profile:
+    return Profile.model_validate(
+        {
+            "id": "profile.modal-gpu",
+            "version": "1.0.0",
+            "extension": "ext.modal",
+        }
+    )
+
+
+def test_provision_and_teardown_with_mock_client(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("MODAL_TOKEN_ID", "modal-token")
+    ext = ModalExtension(
+        {
+            "api_base": "https://api.modal.com",
+            "api_key_env": "MODAL_TOKEN_ID",
+            "provider": "modal",
+        }
+    )
+
+    mock_client = MagicMock()
+    create_resp = MagicMock()
+    create_resp.status_code = 200
+    create_resp.json.return_value = {
+        "sandbox_id": "modal-prov-1",
+        "url": "https://modal.run/sandbox/modal-prov-1",
+        "status": "ready",
+    }
+    ready_resp = MagicMock()
+    ready_resp.status_code = 200
+    ready_resp.json.return_value = {"status": "ready"}
+    delete_resp = MagicMock()
+    delete_resp.status_code = 200
+    mock_client.post.return_value = create_resp
+    mock_client.get.return_value = ready_resp
+    mock_client.delete.return_value = delete_resp
+    mock_client.__enter__.return_value = mock_client
+    mock_client.__exit__.return_value = None
+
+    with patch.object(ext, "_client_factory", return_value=mock_client):
+        handle = ext.provision(_profile(), {}, "modal")
+        assert handle["provider_sandbox_id"] == "modal-prov-1"
+        ext.wait_ready(handle)
+        report = ext.teardown(handle)
+        assert report["provider_removed"] == "true"
+
+
+def test_provision_without_credentials_raises() -> None:
+    ext = ModalExtension({"api_key_env": "MODAL_TOKEN_ID"})
+    with pytest.raises(RuntimeError, match="API key"):
+        ext.provision(_profile(), {}, "modal")