test: wallet rpc basics

[bilthon]

Summary

• add test/functional/feature_wallet_rpc_basics.py as a new functional test focused on basic wallet RPC flow
• register the new test in test/functional/test_runner.py so it can be executed through the standard functional test harness
• establish initial node/test scaffolding to support iterative wallet RPC assertions

Test plan

• cmake -S . -B build
• build/test/functional/test_runner.py feature_wallet_rpc_basics.py
• verify the test passes and appears in the test runner summary