permissions: centralize legacy sandbox projection (#19734)

## Why

The remaining migration work still needs `SandboxPolicy` at a few
compatibility boundaries, but those projections should come from one
canonical path. Keeping ad hoc legacy projections scattered through
app-server, CLI, and config code makes it easy for behavior to drift as
`PermissionProfile` gains fidelity that the legacy enum cannot
represent.

## What Changed

- Adds `Permissions::legacy_sandbox_policy(cwd)` and
`Config::legacy_sandbox_policy()` as the compatibility projection from
the canonical `PermissionProfile`.
- Adds `Permissions::can_set_legacy_sandbox_policy()` so legacy inputs
are checked after they are converted into profile semantics.
- Updates app-server command handling, Windows sandbox setup, session
configuration, and sandbox summaries to use the centralized projection
helper.
- Leaves `SandboxPolicy` in place only for boundary inputs/outputs that
still speak the legacy abstraction.

## Verification

- `cargo check -p codex-config -p codex-core -p codex-sandboxing -p
codex-app-server -p codex-cli -p codex-tui`
- `cargo test -p codex-tui
permissions_selection_history_snapshot_full_access_to_default --
--nocapture`
- `cargo test -p codex-tui
permissions_selection_sends_approvals_reviewer_in_override_turn_context
-- --nocapture`
- `bazel test //codex-rs/tui:tui-unit-tests-bin
--test_arg=permissions_selection_history_snapshot_full_access_to_default
--test_output=errors`
- `bazel test //codex-rs/tui:tui-unit-tests-bin
--test_arg=permissions_selection_sends_approvals_reviewer_in_override_turn_context
--test_output=errors`


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19734).
* #19737
* #19736
* #19735
* __->__ #19734

codex-rs/cli/src/debug_sandbox.rs

@@ -227,7 +227,9 @@ async fn run_command_under_sandbox(
             let args = create_linux_sandbox_command_args_for_policies(
                 command,
                 cwd.as_path(),
-                config.permissions.sandbox_policy.get(),
+                &config
+                    .permissions
+                    .legacy_sandbox_policy(sandbox_policy_cwd.as_path()),
                 &file_system_sandbox_policy,
                 network_sandbox_policy,
                 sandbox_policy_cwd.as_path(),
@@ -290,7 +292,10 @@ async fn run_command_under_windows_session(
     use codex_windows_sandbox::spawn_windows_sandbox_session_elevated;
     use codex_windows_sandbox::spawn_windows_sandbox_session_legacy;
 
-    let policy_str = match serde_json::to_string(config.permissions.sandbox_policy.get()) {
+    let sandbox_policy = config
+        .permissions
+        .legacy_sandbox_policy(sandbox_policy_cwd.as_path());
+    let policy_str = match serde_json::to_string(&sandbox_policy) {
         Ok(policy_str) => policy_str,
         Err(err) => {
             eprintln!("windows sandbox failed to serialize policy: {err}");