Wire the Windows MXC sandbox into command execution (#45176)

## What changed

- Add explicit MXC backend selection and carry its identity through exec-server process reporting and sandbox violation classification.
- Launch MXC through the Codex executable with the effective permission profile and command environment.
- Reject exec-server MXC requests when native MXC is unavailable or when they request a TTY, an `arg0` override, or managed networking. Reject private desktop isolation during MXC preparation.
- Allow an explicitly empty child environment and avoid exposing request payload values in launcher decode errors.

## Testing

Add coverage for sandbox selection and unsupported-request rejection, plus Windows RPC tests for stdin writes and temporary-directory permissions derived from the command environment. Native MXC tests skip when MXC is unavailable.

GitOrigin-RevId: 3626ff0f9ad7f9b812ce09b68c31ea9a5a9c72b1

codex-rs/core/tests/suite/exec.rs

@@ -55,6 +55,7 @@ where
         &cwd,
         std::slice::from_ref(&cwd),
         &None,
+        /*codex_self_exe*/ &None,
         /*use_legacy_landlock*/ false,
         /*stdout_stream*/ None,
     )

codex-rs/core/tests/suite/windows_sandbox.rs

@@ -375,6 +375,7 @@ async fn windows_restricted_token_rejects_exact_and_glob_deny_read_policy() -> a
         &cwd,
         std::slice::from_ref(&cwd),
         &None,
+        /*codex_self_exe*/ &None,
         /*use_legacy_landlock*/ false,
         /*stdout_stream*/ None,
     )
@@ -424,6 +425,7 @@ async fn windows_elevated_does_not_create_missing_workspace_metadata() -> anyhow
         &cwd,
         std::slice::from_ref(&cwd),
         &None,
+        /*codex_self_exe*/ &None,
         /*use_legacy_landlock*/ false,
         /*stdout_stream*/ None,
     )
@@ -603,6 +605,7 @@ async fn windows_elevated_enforces_deny_read_and_protects_setup_marker() -> anyh
         &cwd,
         std::slice::from_ref(&cwd),
         &None,
+        /*codex_self_exe*/ &None,
         /*use_legacy_landlock*/ false,
         /*stdout_stream*/ None,
     )