chore: move pre_main_hardening() utility into its own crate (#4403)

codex-rs/Cargo.lock

@@ -692,12 +692,12 @@ dependencies = [
  "codex-exec",
  "codex-login",
  "codex-mcp-server",
+ "codex-process-hardening",
  "codex-protocol",
  "codex-protocol-ts",
  "codex-responses-api-proxy",
  "codex-tui",
  "ctor 0.5.0",
- "libc",
  "owo-colors",
  "predicates",
  "pretty_assertions",
@@ -949,6 +949,13 @@ dependencies = [
  "wiremock",
 ]
 
+[[package]]
+name = "codex-process-hardening"
+version = "0.0.0"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "codex-protocol"
 version = "0.0.0"