[ISSUE #6292] Avoid mutating divide upstream for specify-domain

[Federicorao]

Summary

• derive the request domain from the specify-domain header without mutating the selected cached Upstream
• add a DividePlugin regression test that verifies the exchange domain changes while the cached upstream URL remains unchanged

Verification

• git diff --check
• attempted ./mvnw -pl shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-divide -Dtest=DividePluginTest -DskipITs test, but the local shell cannot locate a Java runtime

Fixes #6292