* fix: restore offline tiktoken cache for non-root envs
Made-with: Cursor
* chore: mkdir for custom tiktoken cache dir
Made-with: Cursor
* test: patch tiktoken.get_encoding in custom-dir test to avoid network
Made-with: Cursor
* test: clear CUSTOM_TIKTOKEN_CACHE_DIR in helper for test isolation
Made-with: Cursor
* test: restore default_encoding module state after custom-dir test
Made-with: Cursor