permissions: derive snapshot sandbox projections (#19775)

## Why

`ThreadConfigSnapshot` is used by app-server and thread metadata code as
a stable view of active runtime settings. Keeping both `sandbox_policy`
and `permission_profile` in the snapshot duplicates permission state and
makes it possible for the legacy projection to drift from the canonical
profile.

The legacy `sandbox` value is still needed at app-server compatibility
boundaries, so this PR derives it on demand from the snapshot profile
and cwd instead of storing it.

## What Changed

- Removes `ThreadConfigSnapshot.sandbox_policy`.
- Adds `ThreadConfigSnapshot::sandbox_policy()` as a compatibility
projection from `permission_profile` plus `cwd`.
- Updates app-server response/metadata code and tests to call the
projection only where legacy fields still exist.
- Keeps snapshot construction profile-only so split filesystem rules,
disabled enforcement, and external enforcement remain represented by the
canonical profile.

## Verification

- `cargo test -p codex-app-server
thread_response_permission_profile_preserves_enforcement --lib`
- `cargo test -p codex-core
dispatch_reclaims_stale_global_lock_and_starts_consolidation --lib`



































---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19775).
* #19900
* #19899
* #19776
* __->__ #19775

codex-rs/app-server/src/codex_message_processor.rs

@@ -2712,6 +2712,10 @@ impl CodexMessageProcessor {
                 /*has_in_progress_turn*/ false,
             );
 
+            let sandbox = thread_response_sandbox_policy(
+                &config_snapshot.permission_profile,
+                config_snapshot.cwd.as_path(),
+            );
             let permission_profile =
                 thread_response_permission_profile(config_snapshot.permission_profile);
 
@@ -2724,7 +2728,7 @@ impl CodexMessageProcessor {
                 instruction_sources,
                 approval_policy: config_snapshot.approval_policy.into(),
                 approvals_reviewer: config_snapshot.approvals_reviewer.into(),
-                sandbox: config_snapshot.sandbox_policy.into(),
+                sandbox,
                 permission_profile,
                 reasoning_effort: config_snapshot.reasoning_effort,
             };
@@ -3284,7 +3288,7 @@ impl CodexMessageProcessor {
             builder.model_provider = Some(model_provider.clone());
             builder.cwd = config_snapshot.cwd.to_path_buf();
             builder.cli_version = Some(env!("CARGO_PKG_VERSION").to_string());
-            builder.sandbox_policy = config_snapshot.sandbox_policy.clone();
+            builder.sandbox_policy = config_snapshot.sandbox_policy();
             builder.approval_mode = config_snapshot.approval_policy;
             let metadata = builder.build(model_provider.as_str());
             if let Err(err) = state_db_ctx.insert_thread_if_absent(&metadata).await {
@@ -8099,7 +8103,6 @@ async fn handle_pending_thread_resume_request(
         service_tier,
         approval_policy,
         approvals_reviewer,
-        sandbox_policy: _,
         permission_profile,
         cwd,
         reasoning_effort,
@@ -8323,8 +8326,9 @@ fn collect_resume_override_mismatches(
         }
     }
     if let Some(requested_sandbox) = request.sandbox.as_ref() {
+        let active_sandbox = config_snapshot.sandbox_policy();
         let sandbox_matches = matches!(
-            (requested_sandbox, &config_snapshot.sandbox_policy),
+            (requested_sandbox, &active_sandbox),
             (
                 SandboxMode::ReadOnly,
                 codex_protocol::protocol::SandboxPolicy::ReadOnly { .. }
@@ -8341,8 +8345,7 @@ fn collect_resume_override_mismatches(
         );
         if !sandbox_matches {
             mismatch_details.push(format!(
-                "sandbox requested={requested_sandbox:?} active={:?}",
-                config_snapshot.sandbox_policy
+                "sandbox requested={requested_sandbox:?} active={active_sandbox:?}"
             ));
         }
     }
@@ -10042,7 +10045,6 @@ mod tests {
             service_tier: Some(codex_protocol::config_types::ServiceTier::Flex),
             approval_policy: codex_protocol::protocol::AskForApproval::OnRequest,
             approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer::User,
-            sandbox_policy: codex_protocol::protocol::SandboxPolicy::DangerFullAccess,
             permission_profile: codex_protocol::models::PermissionProfile::Disabled,
             cwd,
             ephemeral: false,