Fix incorrect execution of expected-to-fail SQL in mysqltest case mysql8_t_2.regular_expressions_utf-8_icu_67 on Windows

Environment

• Version: Server version: 5.7.25 OceanBase seekdb (r) (Built 1-e1ac5a57bf17078ca46ec9add574edeffac7c3f9 Apr 7 2026)
• Is this an RD-provided temporary version?: No
• Reproducibility: Stable (Consistently Reproducible)

Description

During mysqltest execution on Windows, a specific SQL statement that is expected to fail (and throw an error) is instead executing successfully. This issue is found in the test case mysql8_t_2.regular_expressions_utf-8_icu_67.

Steps to Reproduce

1. Connect to the database using: mysql -uroot -h -P 2881
2. Execute the following SQL statement:

SELECT "hello hello" regexp '\\X' as yesno;

Actual Result:
The query executes successfully and returns a result (e.g., yesno: 0).

Expected Result:
The query should fail and return the error:

ERROR 1139 (42000): Got error 'U_MISSING_RESOURCE_ERROR' from regexp

Impact

This is a test case failure indicating a discrepancy between OceanBase's REGEXP behavior and the expected MySQL-compatible behavior for the \\X (Unicode grapheme cluster) pattern under the ICU 67 library with UTF-8 charset. It affects test suite correctness and may indicate a potential functional bug in the regular expression implementation.

Other Context

This issue was reported from external business operations. Please classify the work item type appropriately based on the actual scenario:

• Customer Service Feedback: Refers to issues found before production deployment or during POC, where a confirmed product bug requires a fix and release.
• Online Issue: A confirmed product bug that has occurred in a live, deployed environment but is not classified as a fault.
• Online Fault: A confirmed product bug in a live environment that has caused business errors or negative impact on the customer side (the definition of a fault is at the discretion of the delivery service team).

Start by running the mysql8_t_2.regular_expressions_utf-8_icu_67 test on Windows and reproducing the SELECT "hello hello" regexp '\X' statement. Trace the REGEXP execution and its ICU 67 handling for UTF-8, then confirm that the expected U_MISSING_RESOURCE_ERROR is returned and the mysqltest case passes.