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/linux-sandbox/src/linux_run_main.rs

@@ -391,7 +391,7 @@ mod tests {
     fn inserts_bwrap_argv0_before_command_separator() {
         let argv = build_bwrap_argv(
             vec!["/bin/true".to_string()],
-            &SandboxPolicy::ReadOnly,
+            &SandboxPolicy::new_read_only_policy(),
             Path::new("/"),
             BwrapOptions {
                 mount_proc: true,
@@ -425,7 +425,7 @@ mod tests {
     fn inserts_unshare_net_when_network_isolation_requested() {
         let argv = build_bwrap_argv(
             vec!["/bin/true".to_string()],
-            &SandboxPolicy::ReadOnly,
+            &SandboxPolicy::new_read_only_policy(),
             Path::new("/"),
             BwrapOptions {
                 mount_proc: true,
@@ -439,7 +439,7 @@ mod tests {
     fn inserts_unshare_net_when_proxy_only_network_mode_requested() {
         let argv = build_bwrap_argv(
             vec!["/bin/true".to_string()],
-            &SandboxPolicy::ReadOnly,
+            &SandboxPolicy::new_read_only_policy(),
             Path::new("/"),
             BwrapOptions {
                 mount_proc: true,