fix(sentinel): serialize global rule updates

[dengliming]

Summary

• serialize Sentinel flow and degrade rule read-modify-write sequences across handler instances
• include cache updates and removals in the same critical section
• add concurrent regression coverage to ensure no global rules are lost

Testing

• mvn -q -pl shenyu-plugin/shenyu-plugin-fault-tolerance/shenyu-plugin-sentinel -am -DskipTests=false -Dcheckstyle.skip=false -Dtest=SentinelRuleHandleTest -DfailIfNoTests=false test

Fixes #6759

[Aias00]

Approved as PMC (Aias00). Reviewed the diff — small, well-scoped fix with regression tests; mergeable.