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/windows-sandbox-rs/src/elevated_impl.rs

@@ -238,9 +238,14 @@ mod windows_impl {
         ) {
             anyhow::bail!("DangerFullAccess and ExternalSandbox are not supported for sandboxing")
         }
+        if !policy.has_full_disk_read_access() {
+            anyhow::bail!(
+                "Restricted read-only access is not yet supported by the Windows sandbox backend"
+            );
+        }
         let caps = load_or_create_cap_sids(codex_home)?;
         let (psid_to_use, cap_sids) = match &policy {
-            SandboxPolicy::ReadOnly => (
+            SandboxPolicy::ReadOnly { .. } => (
                 unsafe { convert_string_sid_to_sid(&caps.readonly).unwrap() },
                 vec![caps.readonly.clone()],
             ),
@@ -469,6 +474,7 @@ mod windows_impl {
         fn workspace_policy(network_access: bool) -> SandboxPolicy {
             SandboxPolicy::WorkspaceWrite {
                 writable_roots: Vec::new(),
+                read_only_access: Default::default(),
                 network_access,
                 exclude_tmpdir_env_var: false,
                 exclude_slash_tmp: false,
@@ -487,7 +493,7 @@ mod windows_impl {
 
         #[test]
         fn applies_network_block_for_read_only() {
-            assert!(!SandboxPolicy::ReadOnly.has_full_network_access());
+            assert!(!SandboxPolicy::new_read_only_policy().has_full_network_access());
         }
     }
 }