fix(server): clean up Windows machine discovery process trees

[wenyue]

Summary

• coalesce identical overlapping flutter attach --machine discovery requests and serialize requests with different inputs
• on Windows, validate the shell wrapper by PID, creation time, executable name, and command line before terminating it through a retained native process handle
• retain descendant identity as PID plus creation time, stop the verified wrapper, then repeatedly rescan and terminate matching Dart descendants until the tree is stably empty
• fail closed on PowerShell/runtime errors; remove the unsafe bare-PID taskkill and direct-kill fallbacks
• preserve the public const FlutterToolMachineDiscovery(...) constructor

Root cause

FlutterToolMachineDiscovery starts Flutter with runInShell: true, so Windows returns a cmd.exe wrapper. Live process tracing showed that taskkill /PID <cmd-pid> /T /F terminated the wrapper but left the dart.exe and dartvm.exe processes running. Repeated discovery therefore accumulated orphaned flutter_tools.snapshot attach --machine processes and memory.

The hardened cleanup snapshots and verifies the exact wrapper identity, pins its native process handle, records descendant identities as PID plus CreationDate, stops the wrapper, and rescans for late-created descendants. The root tombstone is bounded by the verified handle's ExitTime, preventing PID reuse from extending the old tree.

Validation

• Flutter 3.44.8 analyze on the changed source and tests: no issues
• Flutter 3.44.8 focused tests: 17 passed
• steward probe --json --profile quick: all 8 actions passed
• git diff --cached --check
• GitNexus staged impact: medium, limited to four existing machine-discovery flows
• compiled Windows executable and copied it into the local cache with matching SHA-256
• real cached-binary lifecycle smoke: observed 2 attach descendants, cleanup completed in 4453 ms, 0 remaining
• independent staged-diff review: Ready to merge, no remaining Critical or Important findings
• full make check-contracts was not run; the repository-owned quick gate and focused Dart checks above passed

Contributor Checklist

• User-facing change recorded under CHANGELOG.md Unreleased
• No secrets, tokens, or private URLs added
• Existing unrelated worktree changes were not staged or modified

[coderabbitai]

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