test(databus): add unit tests for cache key extraction and singleton

2 tests covering _extract_data_type parsing and get_cache() singleton pattern.

tests/unit/domains/databus/test_cache.py

@@ -0,0 +1,15 @@
+from app.domains.databus.cache import get_cache
+
+
+def test_extract_data_type():
+    c = get_cache()
+    assert c._extract_data_type("source:token_price:abc123") == "token_price"
+    assert c._extract_data_type("databus:news:xyz") == "news"
+    assert c._extract_data_type("nodelimiter") == "default"
+    assert c._extract_data_type("single:") == ""
+
+
+def test_get_cache_is_singleton():
+    c1 = get_cache()
+    c2 = get_cache()
+    assert c1 is c2