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/core/tests/suite/codex_delegate.rs

@@ -64,7 +64,7 @@ async fn codex_delegate_forwards_exec_approval_and_proceeds_on_approval() {
     // routes ExecApprovalRequest via the parent.
     let mut builder = test_codex().with_model("gpt-5.1").with_config(|config| {
         config.approval_policy = Constrained::allow_any(AskForApproval::OnRequest);
-        config.sandbox_policy = Constrained::allow_any(SandboxPolicy::ReadOnly);
+        config.sandbox_policy = Constrained::allow_any(SandboxPolicy::new_read_only_policy());
     });
     let test = builder.build(&server).await.expect("build test codex");
 
@@ -146,7 +146,7 @@ async fn codex_delegate_forwards_patch_approval_and_proceeds_on_decision() {
     let mut builder = test_codex().with_model("gpt-5.1").with_config(|config| {
         config.approval_policy = Constrained::allow_any(AskForApproval::OnRequest);
         // Use a restricted sandbox so patch approval is required
-        config.sandbox_policy = Constrained::allow_any(SandboxPolicy::ReadOnly);
+        config.sandbox_policy = Constrained::allow_any(SandboxPolicy::new_read_only_policy());
         config.include_apply_patch_tool = true;
     });
     let test = builder.build(&server).await.expect("build test codex");