fix(sdk): fence cold-start command generations

[Chamberlain1998]

Summary

• Return an int-compatible tracked process handle carrying the native handle and command generation.
• Serialize explicit reload against command operations with a writer/readers gate.
• Invalidate pre-reload tracked handles and raw integer PIDs only after an authoritative cold-start reload commits.
• Preserve existing handle behavior for snapshot reloads; explicit snapshot-mode native errors remain native errors and do not invalidate the generation.
• Keep conservative first-native-failure invalidation only for older Sandbox SDKs that do not expose a recovery mode.
• Preserve the existing Sandbox, commands, files, shell, and PTY facade objects across reload.

Compatibility

• No public method signature changes.
• Completed command results remain readable after cold start.
• Automatic failover has no synchronous reload-mode response and is therefore not presented as the same client-side generation boundary.

Verification

• Python SDK unit tests: 290/290.
• Ruff passes for the modified backend and tests.
• runsc standalone official integration: 7/7.
• Cold reload invalidates the pending old handle while the same logical Sandbox accepts new commands.
• Snapshot reload restores checkpoint state and re-establishes the reverse tunnel.

Depends on the coordinated YuanRong change: https://gitcode.com/openeuler/yuanrong/merge_requests/1036

Supersedes closed draft #53, which GitHub cannot reopen because its head branch was force-pushed during review fixes.