fix: use random port on each startApp to avoid port binding conflict in DbLiteTest

[vividctrlalt]

Summary

• Fix flaky test DbLiteRocksDbTest.testToolsWithRocksDB caused by TCP port binding conflict
• DbLiteTest.testTools() calls startApp() 4 times in a single test, but the RPC port was only set once in init(). After shutdown(), the port may remain in TCP TIME_WAIT state, causing the next startApp() to fail with Failed to bind to address 0.0.0.0:xxxx
• Fix: call PublicMethod.chooseRandomPort() at the beginning of each startApp() to ensure a fresh port every time

Test plan

• DbLiteRocksDbTest.testToolsWithRocksDB passed 3/3 runs locally
• DbLiteLevelDbTest.testToolsWithLevelDB passed 3/3 runs locally
• DbLiteRocksDbV2Test.testToolsWithRocksDB passed 3/3 runs locally