refactor: update kota/codec includes for new kotatsu header layout

kotatsu moved all codec headers into subdirectories; top-level files
like codec/raw_value.h and codec/toml.h no longer exist.

- codec/raw_value.h → removed (re-exported by json/json.h and
  bincode/bincode.h)
- codec/toml.h → codec/toml/toml.h
- codec/json/serializer.h → codec/json/json.h (use umbrella header)

Made-with: Cursor

tests/unit/server/stateful_worker_tests.cpp

@@ -5,7 +5,7 @@
 #include "server/protocol.h"
 #include "server/worker_test_helpers.h"
 
-#include "kota/codec/raw_value.h"
+#include "kota/codec/json/json.h"
 
 namespace clice::testing {