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

code/tests/test_random_permalink_roundtrip.py

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+import os
+import base64
+import json
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+@pytest.fixture(scope="module")
+def client():
+    # Ensure flags and frozen dataset
+    os.environ["RANDOM_MODES"] = "1"
+    os.environ["RANDOM_UI"] = "1"
+    os.environ["CSV_FILES_DIR"] = os.path.join("csv_files", "testdata")
+
+    from web.app import app
+
+    with TestClient(app) as c:
+        yield c
+
+
+def _decode_state_token(token: str) -> dict:
+    pad = "=" * (-len(token) % 4)
+    raw = base64.urlsafe_b64decode((token + pad).encode("ascii")).decode("utf-8")
+    return json.loads(raw)
+
+
+def test_permalink_roundtrip_via_build_routes(client: TestClient):
+    # Create a permalink via random full build
+    r1 = client.post("/api/random_full_build", json={"seed": 777})
+    assert r1.status_code == 200, r1.text
+    p1 = r1.json().get("permalink")
+    assert p1 and p1.startswith("/build/from?state=")
+    token = p1.split("state=", 1)[1]
+    state1 = _decode_state_token(token)
+    rnd1 = state1.get("random") or {}
+
+    # Visit the permalink (server should rehydrate session from token)
+    r_page = client.get(p1)
+    assert r_page.status_code == 200
+
+    # Ask server to produce a permalink from current session
+    r2 = client.get("/build/permalink")
+    assert r2.status_code == 200, r2.text
+    body2 = r2.json()
+    assert body2.get("ok") is True
+    p2 = body2.get("permalink")
+    assert p2 and p2.startswith("/build/from?state=")
+    token2 = p2.split("state=", 1)[1]
+    state2 = _decode_state_token(token2)
+    rnd2 = state2.get("random") or {}
+
+    # The random payload should survive the roundtrip unchanged
+    assert rnd2 == rnd1