fix: cleanup the contract of the general-purpose exec() function (#17870)

`exec()` had a number of arguments that were unused, making the function
signature misleading. This PR aims to clean things up to clarify the
role of this function and to clarify which fields of `ExecParams` are
unused and why.

codex-rs/core/src/exec_tests.rs

@@ -277,10 +277,6 @@ async fn exec_full_buffer_capture_ignores_expiration() -> Result<()> {
             justification: None,
             arg0: None,
         },
-        SandboxType::None,
-        &SandboxPolicy::DangerFullAccess,
-        &FileSystemSandboxPolicy::unrestricted(),
-        /*windows_sandbox_filesystem_overrides*/ None,
         NetworkSandboxPolicy::Enabled,
         /*stdout_stream*/ None,
         /*after_spawn*/ None,
@@ -317,10 +313,6 @@ async fn exec_full_buffer_capture_keeps_io_drain_timeout_when_descendant_holds_p
                 justification: None,
                 arg0: None,
             },
-            SandboxType::None,
-            &SandboxPolicy::DangerFullAccess,
-            &FileSystemSandboxPolicy::unrestricted(),
-            /*windows_sandbox_filesystem_overrides*/ None,
             NetworkSandboxPolicy::Enabled,
             /*stdout_stream*/ None,
             /*after_spawn*/ None,
@@ -931,10 +923,6 @@ async fn kill_child_process_group_kills_grandchildren_on_timeout() -> Result<()>
 
     let output = exec(
         params,
-        SandboxType::None,
-        &SandboxPolicy::new_read_only_policy(),
-        &FileSystemSandboxPolicy::from(&SandboxPolicy::new_read_only_policy()),
-        /*windows_sandbox_filesystem_overrides*/ None,
         NetworkSandboxPolicy::Restricted,
         /*stdout_stream*/ None,
         /*after_spawn*/ None,