Add pytest suite and GitHub Actions CI

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up pixi
+        uses: prefix-dev/setup-pixi@v0.8.8
+        with:
+          manifest-path: pixi.toml
+          locked: true
+
+      - name: Run tests
+        run: pixi run test

conftest.py

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+"""Shared fixtures: fake Venmo client/transaction objects so tests never touch the network."""
+
+from types import SimpleNamespace
+
+import pytest
+
+
+def make_user(user_id, username="user", display_name="User"):
+    return SimpleNamespace(id=user_id, username=username, display_name=display_name)
+
+
+def make_txn(
+    *,
+    txn_id="1",
+    payment_type="pay",
+    amount=10.0,
+    note="note",
+    status="settled",
+    audience="private",
+    date_created="1780887063",
+    actor=None,
+    target=None,
+):
+    return SimpleNamespace(
+        id=txn_id,
+        payment_type=payment_type,
+        amount=amount,
+        note=note,
+        status=status,
+        audience=audience,
+        date_created=date_created,
+        actor=actor,
+        target=target,
+    )
+
+
+class FakeUserApi:
+    """Records which read method was called and returns a canned page (a plain list)."""
+
+    def __init__(self, txns):
+        self._txns = txns
+        self.calls = []
+
+    def get_user_transactions(self, user_id=None, limit=50, before_id=None):
+        self.calls.append(
+            {"method": "get_user_transactions", "user_id": user_id, "limit": limit, "before_id": before_id}
+        )
+        return list(self._txns)
+
+    def get_transaction_between_two_users(
+        self, user_id_one=None, user_id_two=None, limit=50, before_id=None
+    ):
+        self.calls.append(
+            {
+                "method": "get_transaction_between_two_users",
+                "user_id_one": user_id_one,
+                "user_id_two": user_id_two,
+                "limit": limit,
+                "before_id": before_id,
+            }
+        )
+        return list(self._txns)
+
+
+class FakeClient:
+    def __init__(self, txns, my_id):
+        self.user = FakeUserApi(txns)
+        self._my_id = my_id
+
+    def my_profile(self):
+        return SimpleNamespace(id=self._my_id)
+
+
+@pytest.fixture
+def fake_client_factory(monkeypatch):
+    """Patch server.get_client to return a FakeClient; hand the client back for assertions."""
+    import server
+
+    def _factory(txns, my_id="me"):
+        client = FakeClient(txns, my_id)
+        monkeypatch.setattr(server, "get_client", lambda: client)
+        return client
+
+    return _factory