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/command_runner_win.rs

@@ -114,6 +114,11 @@ pub fn main() -> Result<()> {
     );
 
     let policy = parse_policy(&req.policy_json_or_preset).context("parse policy_json_or_preset")?;
+    if !policy.has_full_disk_read_access() {
+        anyhow::bail!(
+            "Restricted read-only access is not yet supported by the Windows sandbox backend"
+        );
+    }
     let mut cap_psids: Vec<*mut c_void> = Vec::new();
     for sid in &req.cap_sids {
         let Some(psid) = (unsafe { convert_string_sid_to_sid(sid) }) else {
@@ -129,7 +134,9 @@ pub fn main() -> Result<()> {
     let base = unsafe { get_current_token_for_restriction()? };
     let token_res: Result<HANDLE> = unsafe {
         match &policy {
-            SandboxPolicy::ReadOnly => create_readonly_token_with_caps_from(base, &cap_psids),
+            SandboxPolicy::ReadOnly { .. } => {
+                create_readonly_token_with_caps_from(base, &cap_psids)
+            }
             SandboxPolicy::WorkspaceWrite { .. } => {
                 create_workspace_write_token_with_caps_from(base, &cap_psids)
             }