## Why
`//codex-rs/shell-command:shell-command-unit-tests` became a real
bottleneck in the Windows Bazel lane because repeated calls to
`is_safe_command_windows()` were starting a fresh PowerShell parser
process for every `powershell.exe -Command ...` assertion.
PR #16056 was motivated by that same bottleneck, but its test-only
shortcut was the wrong layer to optimize because it weakened the
end-to-end guarantee that our runtime path really asks PowerShell to
parse the command the way we expect.
This PR attacks the actual cost center instead: it keeps the real
PowerShell parser in the loop, but turns that parser into a long-lived
helper process so both tests and the runtime safe-command path can reuse
it across many requests.
## What Changed
- add `shell-command/src/command_safety/powershell_parser.rs`, which
keeps one mutex-protected parser process per PowerShell executable path
and speaks a simple JSON-over-stdio request/response protocol
- turn `shell-command/src/command_safety/powershell_parser.ps1` into a
long-running parser server with comments explaining the protocol, the
AST-shape restrictions, and why unsupported constructs are rejected
conservatively
- keep request ids and a one-time respawn path so a dead or
desynchronized cached child fails closed instead of silently returning
mixed parser output
- preserve separate parser processes for `powershell.exe` and
`pwsh.exe`, since they do not accept the same language surface
- avoid a direct `PipelineChainAst` type reference in the PowerShell
script so the parser service still runs under Windows PowerShell 5.1 as
well as newer `pwsh`
- make `shell-command/src/command_safety/windows_safe_commands.rs`
delegate to the new parser utility instead of spawning a fresh
PowerShell process for every parse
- add a Windows-only unit test that exercises multiple sequential
requests against the same parser process
## Testing
- adds a Windows-only parser-reuse unit test in `powershell_parser.rs`
- the main end-to-end verification for this change is the Windows CI
lane, because the new service depends on real `powershell.exe` /
`pwsh.exe` behavior
Michael Bolin