fix(ai): evict transformer clients by rule id

[dengliming]

Summary

• evict request-transformer clients with the same rule ID used for insertion
• apply the same cache-key fix to response-transformer clients
• remove handle parsing from client cleanup and add cache eviction regression coverage

Testing

• mvn -q -pl shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-request-transformer,shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-response-transformer -am -DskipTests=false -Dcheckstyle.skip=false -Dtest=AiRequestTransformerPluginHandlerTest,AiResponseTransformerPluginHandlerTest -DfailIfNoTests=false test

Fixes #6755

[Aias00]

Approved as PMC (Aias00). Reviewed the diff — small, well-scoped fix with regression tests; mergeable. Note: the only failing CI checks (shenyu-integrated-test-*/e2e-case-logging-rocketmq) are failing cluster-wide across unrelated PRs (shared CI-env issue), not caused by this change.