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/app-server/src/codex_message_processor.rs

@@ -2307,7 +2307,11 @@ impl CodexMessageProcessor {
                 }
             }
         } else if let Some(policy) = sandbox_policy.map(|policy| policy.to_core()) {
-            match self.config.permissions.sandbox_policy.can_set(&policy) {
+            match self
+                .config
+                .permissions
+                .can_set_legacy_sandbox_policy(&policy, &sandbox_cwd)
+            {
                 Ok(()) => {
                     let file_system_sandbox_policy =
                         codex_protocol::permissions::FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(&policy, &sandbox_cwd);
@@ -8705,7 +8709,9 @@ impl CodexMessageProcessor {
                 Ok(config) => {
                     let setup_request = WindowsSandboxSetupRequest {
                         mode,
-                        policy: config.permissions.sandbox_policy.get().clone(),
+                        policy: config
+                            .permissions
+                            .legacy_sandbox_policy(config.cwd.as_path()),
                         policy_cwd: config.cwd.to_path_buf(),
                         command_cwd,
                         env_map: std::env::vars().collect(),