Tighten the PowerShell tool to surface a clear not-found error when neither pwsh nor powershell exists, and mark explicit background execution as user-requested in the returned metadata. Harden the PowerShell tests against PATH mutation races while keeping the change confined to the tools crate.\n\nConstraint: Must not touch unrelated dirty api files in this worktree\nConstraint: Keep the change limited to rust/crates/tools\nRejected: Broader shell abstraction cleanup | not needed for this parity slice\nConfidence: high\nScope-risk: narrow\nReversibility: clean\nDirective: Keep PowerShell output metadata aligned with bash semantics when adding future shell parity improvements\nTested: cargo test -p tools\nNot-tested: real powershell.exe behavior on Windows hosts
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