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/protocol/src/models.rs

@@ -312,7 +312,7 @@ impl DeveloperInstructions {
 
         let (sandbox_mode, writable_roots) = match sandbox_policy {
             SandboxPolicy::DangerFullAccess => (SandboxMode::DangerFullAccess, None),
-            SandboxPolicy::ReadOnly => (SandboxMode::ReadOnly, None),
+            SandboxPolicy::ReadOnly { .. } => (SandboxMode::ReadOnly, None),
             SandboxPolicy::ExternalSandbox { .. } => (SandboxMode::DangerFullAccess, None),
             SandboxPolicy::WorkspaceWrite { .. } => {
                 let roots = sandbox_policy.get_writable_roots_with_cwd(cwd);
@@ -1223,6 +1223,7 @@ mod tests {
     fn builds_permissions_from_policy() {
         let policy = SandboxPolicy::WorkspaceWrite {
             writable_roots: vec![],
+            read_only_access: Default::default(),
             network_access: true,
             exclude_tmpdir_env_var: false,
             exclude_slash_tmp: false,