[Studio] Add proxy address removal flow

[Aias00]

Summary

• add a form-encoded /api/proxy/removeProxyAddr.do compatibility endpoint and service logic
• wire the Studio Proxy page remove action to the backend and refresh the node list after success
• cover backend removal semantics and frontend API request encoding

Validation

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q -Dtest=ProxyCompatControllerTest,ProxyAddressServiceTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q test (50 surefire files, 298 tests, 0 failures, 0 errors)
• npm test -- --run src/api/proxy.test.ts
• npm test (36 files, 164 tests)
• npm run lint (0 errors, 4 existing fast-refresh warnings)
• npm run build
• git diff --check

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