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_e2b.py

@@ -0,0 +1,72 @@
+"""E2B cloud adapter tests."""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from sandboxer.extensions.credentials import credentials_available, resolve_api_key
+from sandboxer.extensions.e2b import E2BExtension
+from sandboxer.models import Profile
+
+
+def _profile() -> Profile:
+    return Profile.model_validate(
+        {
+            "id": "profile.e2b-burst",
+            "version": "1.0.0",
+            "extension": "ext.e2b",
+        }
+    )
+
+
+def test_credentials_from_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("E2B_API_KEY", "test-key")
+    config = {"api_key_env": "E2B_API_KEY", "secret_ref": "e2b-api-key"}
+    assert resolve_api_key(config, extension_id="ext.e2b") == "test-key"
+    assert credentials_available("ext.e2b", config)
+
+
+def test_provision_and_teardown_with_mock_client(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("E2B_API_KEY", "test-key")
+    ext = E2BExtension(
+        {
+            "api_base": "https://api.e2b.dev",
+            "api_key_env": "E2B_API_KEY",
+            "provider": "e2b",
+            "template_id": "base",
+        }
+    )
+
+    mock_client = MagicMock()
+    create_resp = MagicMock()
+    create_resp.status_code = 200
+    create_resp.json.return_value = {
+        "sandboxID": "e2b-prov-1",
+        "sandboxURL": "https://e2b-prov-1.e2b.dev",
+    }
+    ready_resp = MagicMock()
+    ready_resp.status_code = 200
+    delete_resp = MagicMock()
+    delete_resp.status_code = 204
+    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(), {}, "e2b")
+        assert handle["provider_sandbox_id"] == "e2b-prov-1"
+        reach = ext.wait_ready(handle)
+        assert "e2b-prov-1" in reach["endpoint"]
+        report = ext.teardown(handle)
+        assert report["provider_removed"] == "true"
+
+
+def test_estimate_cost() -> None:
+    ext = E2BExtension({"rate_usd_per_hour": 0.15, "session_fee_usd": 0.02})
+    quote = ext.estimate_cost(_profile(), {}, duration_s=3600)
+    assert quote.extension_id == "ext.e2b"
+    assert quote.estimated_usd > 0