util: Use job objects to reap spawned process trees on Windows

[miguelraz]

On Windows, util::process::Child::kill() only terminated the direct child process, and nothing tied the lifetime of spawned process trees to Zed. External agent servers launched through ACP (e.g. claude-code-acp via npx) spawn node workers and MCP servers as grandchildren, which were orphaned on every session teardown and accumulated indefinitely — hundreds of idle node.exe processes and GBs of RAM over days.

This PR assigns spawned processes to a Win32 job object configured with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE:

• kill() now calls TerminateJobObject, killing the entire tree instead of just the (shell wrapper) child.
• Dropping Child closes the job handle, which makes the OS reap the tree — including when Zed exits for any reason (even crashes), since the OS closes its handles.
• Unix behavior (process groups via killpg) is unchanged.

Added two Windows tests that spawn a real powershell -> ping process tree and assert the grandchild is terminated on kill() and on drop. Both fail without the fix and pass with it. Verified with cargo test -p util, script/clippy -p util, and cargo check -p agent_servers -p dap on Windows 11.

Closes #58873

Release Notes:

• Fixed external agent servers and debug adapters leaking helper processes (e.g. node workers and MCP servers) on Windows.

[smitbarmase]

Okay here I got the reproduction:

1. settings.json - add a leaking agent (the modern default self cleans, so you must force the old adapter):

"agent_servers": {
  "Claude Code (leak repro)": {
    "command": "npx",
    "args": ["-y", "@zed-industries/claude-code-acp@0.16.2"],
    "env": {}
  }
}

2. Also have some mcp server by having .mcp.json in repo root:

{ "mcpServers": { "filesystem": {
  "command": "npx",
  "args": ["-y", "@modelcontextprotocol/server-filesystem", "C:\\Users\\heysm"] } } }

2. Monitor command (separate PowerShell, leave running):

while ($true) { Clear-Host; Get-CimInstance Win32_Process -Filter "Name='node.exe'" |
  Where-Object { $_.CommandLine -match 'claude-code-acp|claude-agent-sdk|server-filesystem' } |
  Select ProcessId, CommandLine | Format-List; Start-Sleep 2 }

3. Steps: Open Zed → Agent panel → pick "Claude Code (leak repro)" → send one message → confirm the tree spawns (npx → …claude-code-acp\dist\index.js → …claude-agent-sdk\cli.js → …server-filesystem) → quit Zed.

4. After close, check if you still have running processes. With above setup you should see leaky processes, while with this fix, Zed cleans it up and does not depend on external process for that clean up.

[bennetbo]

Seems reasonable, but I don't have a Windows machine to test nor knowledge of the APIs. Would be good if someone that worked on the Windows port looks at this 😄

[Veykril]

Seems correct to me