fix(vm): clear leftover thread-local VMConfig snapshot

[vividctrlalt]

Summary

Fix intermittent :framework:test failures on Rocky Linux JDK 8 after tronprotocol#6857.

The same commit is green on Debian 11 JDK 8, Ubuntu, and macOS. London / Osaka tests fail because initAllowTvm*() writes globalSnapshot while current() still reads a leftover thread-local snapshot. Assertions are unchanged.

Root cause

tronprotocol#6857 split VM config into a process-global snapshot (block processing) and a thread-local snapshot (constant calls). Getters use current() (local if present). initAllowTvm*() still mutates only the global snapshot. ConfigLoader.disable skips load(), so a leftover local view is never cleared. With forkEvery = 100, the next test on the same thread does not observe the flags it just set.

VMConfigIsolationTest does not cover this contract. Pre-tronprotocol#6857 tests (AllowTvmLondonTest, ValidateMultiSignContractTest) still use init*.

Rocky Linux reproduces it more often because the host is slower. A failed-job rerun going green is expected.

Separate races on the same host:

• BackupServer.close() could run before bind and called stop() before closing the UDP channel (channel was not volatile; port probe was TCP-only).
• TransactionsMsgHandlerTest used when(mock.foo()) while handleTransaction called isBadPeer() on the same mock.

Changes

• ConfigLoader.load: when disable is set, clearLocalSnapshot() so init* is visible to current().
• Base test teardown: drop the local snapshot and reset ConfigLoader.disable.
• London / Osaka tests: persist the flag to the store and clear the local snapshot.
• BackupServer: volatile channel, close the channel before backupManager.stop(), close-if-shutdown after bind; wait for bind instead of sleep(1000); chooseRandomPort probes UDP.
• TransactionsMsgHandlerTest: doReturn and a fresh peer mock per case.

Related

tronprotocol#6857

Test plan

• Rocky Linux JDK 8 ./gradlew :framework:test --tests org.tron.common.runtime.vm.AllowTvmLondonTest --tests org.tron.common.runtime.vm.ValidateMultiSignContractTest --tests org.tron.common.backup.BackupServerTest --tests org.tron.core.net.messagehandler.TransactionsMsgHandlerTest
• Debian 11 JDK 8 still green
• Full :framework:test on Rocky Linux JDK 8