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/core/tests/suite/prompt_caching.rs

@@ -375,6 +375,7 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() -> an
     let writable = TempDir::new().unwrap();
     let new_policy = SandboxPolicy::WorkspaceWrite {
         writable_roots: vec![writable.path().try_into().unwrap()],
+        read_only_access: Default::default(),
         network_access: true,
         exclude_tmpdir_env_var: true,
         exclude_slash_tmp: true,
@@ -618,6 +619,7 @@ async fn per_turn_overrides_keep_cached_prefix_and_key_constant() -> anyhow::Res
     let writable = TempDir::new().unwrap();
     let new_policy = SandboxPolicy::WorkspaceWrite {
         writable_roots: vec![AbsolutePathBuf::try_from(writable.path()).unwrap()],
+        read_only_access: Default::default(),
         network_access: true,
         exclude_tmpdir_env_var: true,
         exclude_slash_tmp: true,