CREATE FUNCTION: test not covered for `CREATE FUNCTION ... OPTIONS (FILE = 'libxxx.so')`

Current UTs for **CREATE FUNCTION** only covered `FILE = `, while the production advised usage: `FILE = ` is not covered, neither for online or offline.

Tests should covered for the case: `FILE = `, where `` do not contains any slash(`/`) character, is able to register the external function whose shared library located in `/../udf` or `/../udf`

Look for existing unit tests for CREATE FUNCTION, likely in test files related to SQL or UDFs. The issue is about adding test coverage for the `FILE = 'libxxx.so'` syntax without slashes. Find where the UDF loading logic resolves filenames to paths in tablet_bin_dir/../udf or taskmanager_bin_dir/../udf. Write a test that creates a function with a simple filename and verifies it registers correctly.

索引モデルが issue の本文から書いたものです。