fix: cleanup MCP processes when CLI closes (#2469)

Co-authored-by: Alice Hau <ahau@squareup.com>

Cargo.lock

@@ -2485,6 +2485,7 @@ dependencies = [
  "mcp-core",
  "mcp-server",
  "minijinja",
+ "nix 0.30.1",
  "once_cell",
  "rand",
  "regex",
@@ -3488,9 +3489,9 @@ checksum = "03087c2bad5e1034e8cace5926dec053fb3790248370865f5117a7d0213354c8"
 
 [[package]]
 name = "libc"
-version = "0.2.170"
+version = "0.2.172"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "875b3680cb2f8f71bdcf9a30f38d48282f5d3c95cbf9b3fa57269bb5d5c06828"
+checksum = "d750af042f7ef4f724306de029d18836c26c1765a54a6a3f094cbd23a7267ffa"
 
 [[package]]
 name = "libdbus-sys"
@@ -3700,6 +3701,7 @@ dependencies = [
  "eventsource-client",
  "futures",
  "mcp-core",
+ "nix 0.30.1",
  "rand",
  "reqwest 0.11.27",
  "serde",
@@ -3926,6 +3928,18 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "nix"
+version = "0.30.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "74523f3a35e05aba87a1d978330aef40f67b0304ac79c1c00b294c9830543db6"
+dependencies = [
+ "bitflags 2.9.0",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "nom"
 version = "7.1.3"