Update guardian output schema (#17061)

## Summary
- Update guardian output schema to separate risk, authorization,
outcome, and rationale.
- Feed guardian rationale into rejection messages.
- Split the guardian policy into template and tenant-config sections.

## Validation
- `cargo test -p codex-core mcp_tool_call`
- `env -u CODEX_SANDBOX_NETWORK_DISABLED INSTA_UPDATE=always cargo test
-p codex-core guardian::`

---------

Co-authored-by: Owen Lin <owen@openai.com>

codex-rs/core/src/codex.rs

@@ -2004,6 +2004,7 @@ impl Session {
             session_telemetry,
             models_manager: Arc::clone(&models_manager),
             tool_approvals: Mutex::new(ApprovalStore::default()),
+            guardian_rejection_rationales: Mutex::new(HashMap::new()),
             skills_manager,
             plugins_manager: Arc::clone(&plugins_manager),
             mcp_manager: Arc::clone(&mcp_manager),