Make decorator responsibe for applying the prefix

The key builder should not have to fetch the prefix separately, as this
makes creating custom key builders that much harder.

fastapi_cache/decorator.py

@@ -121,6 +121,7 @@ def cache(
             ) or not FastAPICache.get_enable():
                 return await ensure_async_func(*args, **kwargs)
 
+            prefix = FastAPICache.get_prefix()
             coder = coder or FastAPICache.get_coder()
             expire = expire or FastAPICache.get_expire()
             key_builder = key_builder or FastAPICache.get_key_builder()
@@ -128,7 +129,7 @@ def cache(
 
             cache_key = key_builder(
                 func,
-                namespace,
+                f"{prefix}:{namespace}",
                 request=request,
                 response=response,
                 args=args,

fastapi_cache/key_builder.py

@@ -13,13 +13,7 @@ def default_key_builder(
     args: Optional[tuple[Any, ...]] = None,
     kwargs: Optional[dict[str, Any]] = None,
 ) -> str:
-    from fastapi_cache import FastAPICache
-
-    prefix = f"{FastAPICache.get_prefix()}:{namespace}:"
-    cache_key = (
-        prefix
-        + hashlib.md5(  # nosec:B303
-            f"{func.__module__}:{func.__name__}:{args}:{kwargs}".encode()
-        ).hexdigest()
-    )
-    return cache_key
+    cache_key = hashlib.md5(  # nosec:B303
+        f"{func.__module__}:{func.__name__}:{args}:{kwargs}".encode()
+    ).hexdigest()
+    return f"{namespace}:{cache_key}"