feat: make sandbox read access configurable with ReadOnlyAccess (#11387)

`SandboxPolicy::ReadOnly` previously implied broad read access and could
not express a narrower read surface.
This change introduces an explicit read-access model so we can support
user-configurable read restrictions in follow-up work, while preserving
current behavior today.

It also ensures unsupported backends fail closed for restricted-read
policies instead of silently granting broader access than intended.

## What

- Added `ReadOnlyAccess` in protocol with:
  - `Restricted { include_platform_defaults, readable_roots }`
  - `FullAccess`
- Updated `SandboxPolicy` to carry read-access configuration:
  - `ReadOnly { access: ReadOnlyAccess }`
  - `WorkspaceWrite { ..., read_only_access: ReadOnlyAccess }`
- Preserved existing behavior by defaulting current construction paths
to `ReadOnlyAccess::FullAccess`.
- Threaded the new fields through sandbox policy consumers and call
sites across `core`, `tui`, `linux-sandbox`, `windows-sandbox`, and
related tests.
- Updated Seatbelt policy generation to honor restricted read roots by
emitting scoped read rules when full read access is not granted.
- Added fail-closed behavior on Linux and Windows backends when
restricted read access is requested but not yet implemented there
(`UnsupportedOperation`).
- Regenerated app-server protocol schema and TypeScript artifacts,
including `ReadOnlyAccess`.

## Compatibility / rollout

- Runtime behavior remains unchanged by default (`FullAccess`).
- API/schema changes are in place so future config wiring can enable
restricted read access without another policy-shape migration.

codex-rs/exec-server/tests/common/lib.rs

@@ -88,7 +88,7 @@ where
     S: Service<RoleClient> + ClientHandler,
 {
     let sandbox_state = SandboxState {
-        sandbox_policy: SandboxPolicy::ReadOnly,
+        sandbox_policy: SandboxPolicy::new_read_only_policy(),
         codex_linux_sandbox_exe,
         sandbox_cwd: sandbox_cwd.as_ref().to_path_buf(),
         use_linux_sandbox_bwrap: false,
@@ -110,6 +110,7 @@ where
             // Note that sandbox_cwd will already be included as a writable root
             // when the sandbox policy is expanded.
             writable_roots: vec![],
+            read_only_access: Default::default(),
             network_access: false,
             // Disable writes to temp dir because this is a test, so
             // writable_folder is likely also under /tmp and we want to be