feat(random): multi-theme groundwork, locked reroll export parity, duplicate export fix, expanded diagnostics and test coverage

code/tests/test_random_fallback_and_constraints.py

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+from __future__ import annotations
+
+import importlib
+import os
+from starlette.testclient import TestClient
+
+
+def _mk_client(monkeypatch):
+    monkeypatch.setenv("RANDOM_MODES", "1")
+    monkeypatch.setenv("CSV_FILES_DIR", os.path.join("csv_files", "testdata"))
+    app_module = importlib.import_module('code.web.app')
+    return TestClient(app_module.app)
+
+
+def test_invalid_theme_triggers_fallback_and_echoes_original_theme(monkeypatch):
+    client = _mk_client(monkeypatch)
+    payload = {"seed": 777, "theme": "this theme does not exist"}
+    r = client.post('/api/random_full_build', json=payload)
+    assert r.status_code == 200
+    data = r.json()
+    # Fallback flag should be set with original_theme echoed
+    assert data.get("fallback") is True
+    assert data.get("original_theme") == payload["theme"]
+    # Theme is still the provided theme (we indicate fallback via the flag)
+    assert data.get("theme") == payload["theme"]
+    # Commander/decklist should be present
+    assert isinstance(data.get("commander"), str) and data["commander"]
+    assert isinstance(data.get("decklist"), list)
+
+
+def test_constraints_impossible_returns_422_with_detail(monkeypatch):
+    client = _mk_client(monkeypatch)
+    # Set an unrealistically high requirement to force impossible constraint
+    payload = {"seed": 101, "constraints": {"require_min_candidates": 1000000}}
+    r = client.post('/api/random_full_build', json=payload)
+    assert r.status_code == 422
+    data = r.json()
+    # Structured error payload
+    assert data.get("status") == 422
+    detail = data.get("detail")
+    assert isinstance(detail, dict)
+    assert detail.get("error") == "constraints_impossible"
+    assert isinstance(detail.get("pool_size"), int)