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/core/src/codex_thread.rs

@@ -47,7 +47,6 @@ pub struct ThreadConfigSnapshot {
     pub service_tier: Option<ServiceTier>,
     pub approval_policy: AskForApproval,
     pub approvals_reviewer: ApprovalsReviewer,
-    pub sandbox_policy: SandboxPolicy,
     pub permission_profile: PermissionProfile,
     pub cwd: AbsolutePathBuf,
     pub ephemeral: bool,
@@ -56,6 +55,18 @@ pub struct ThreadConfigSnapshot {
     pub session_source: SessionSource,
 }
 
+impl ThreadConfigSnapshot {
+    pub fn sandbox_policy(&self) -> SandboxPolicy {
+        let file_system_sandbox_policy = self.permission_profile.file_system_sandbox_policy();
+        codex_sandboxing::compatibility_sandbox_policy_for_permission_profile(
+            &self.permission_profile,
+            &file_system_sandbox_policy,
+            self.permission_profile.network_sandbox_policy(),
+            self.cwd.as_path(),
+        )
+    }
+}
+
 /// Turn context overrides that app-server validates before starting a turn.
 #[derive(Clone, Default)]
 pub struct CodexThreadTurnContextOverrides {