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/app-server-test-client/src/lib.rs

@@ -46,6 +46,7 @@ use codex_app_server_protocol::ModelListParams;
 use codex_app_server_protocol::ModelListResponse;
 use codex_app_server_protocol::NewConversationParams;
 use codex_app_server_protocol::NewConversationResponse;
+use codex_app_server_protocol::ReadOnlyAccess;
 use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::SandboxPolicy;
 use codex_app_server_protocol::SendUserMessageParams;
@@ -301,7 +302,9 @@ fn trigger_cmd_approval(
         config_overrides,
         message,
         Some(AskForApproval::OnRequest),
-        Some(SandboxPolicy::ReadOnly),
+        Some(SandboxPolicy::ReadOnly {
+            access: ReadOnlyAccess::FullAccess,
+        }),
         dynamic_tools,
     )
 }
@@ -320,7 +323,9 @@ fn trigger_patch_approval(
         config_overrides,
         message,
         Some(AskForApproval::OnRequest),
-        Some(SandboxPolicy::ReadOnly),
+        Some(SandboxPolicy::ReadOnly {
+            access: ReadOnlyAccess::FullAccess,
+        }),
         dynamic_tools,
     )
 }