permissions: remove legacy read-only access modes (#19449)

## Why

`ReadOnlyAccess` was a transitional legacy shape on `SandboxPolicy`:
`FullAccess` meant the historical read-only/workspace-write modes could
read the full filesystem, while `Restricted` tried to carry partial
readable roots. The partial-read model now belongs in
`FileSystemSandboxPolicy` and `PermissionProfile`, so keeping it on
`SandboxPolicy` makes every legacy projection reintroduce lossy
read-root bookkeeping and creates unnecessary noise in the rest of the
permissions migration.

This PR makes the legacy policy model narrower and explicit:
`SandboxPolicy::ReadOnly` and `SandboxPolicy::WorkspaceWrite` represent
the old full-read sandbox modes only. Split readable roots, deny-read
globs, and platform-default/minimal read behavior stay in the runtime
permissions model.

## What changed

- Removes `ReadOnlyAccess` from
`codex_protocol::protocol::SandboxPolicy`, including the generated
`access` and `readOnlyAccess` API fields.
- Updates legacy policy/profile conversions so restricted filesystem
reads are represented only by `FileSystemSandboxPolicy` /
`PermissionProfile` entries.
- Keeps app-server v2 compatible with legacy `fullAccess` read-access
payloads by accepting and ignoring that no-op shape, while rejecting
legacy `restricted` read-access payloads instead of silently widening
them to full-read legacy policies.
- Carries Windows sandbox platform-default read behavior with an
explicit override flag instead of depending on
`ReadOnlyAccess::Restricted`.
- Refreshes generated app-server schema/types and updates tests/docs for
the simplified legacy policy shape.

## Verification

- `cargo check -p codex-app-server-protocol --tests`
- `cargo check -p codex-windows-sandbox --tests`
- `cargo test -p codex-app-server-protocol sandbox_policy_`


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19449).
* #19395
* #19394
* #19393
* #19392
* #19391
* __->__ #19449

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -484,7 +484,6 @@ fn summarize_sandbox_policy(sandbox_policy: &SandboxPolicy) -> String {
             network_access,
             exclude_tmpdir_env_var,
             exclude_slash_tmp,
-            read_only_access: _,
         } => {
             let mut summary = "workspace-write".to_string();
             let mut writable_entries = vec!["workdir".to_string()];

codex-rs/exec/src/lib_tests.rs

@@ -422,7 +422,6 @@ fn session_configured_from_thread_response_uses_review_policy_from_response() {
         approvals_reviewer: codex_app_server_protocol::ApprovalsReviewer::AutoReview,
         sandbox: codex_app_server_protocol::SandboxPolicy::WorkspaceWrite {
             writable_roots: vec![],
-            read_only_access: codex_app_server_protocol::ReadOnlyAccess::FullAccess,
             network_access: false,
             exclude_tmpdir_env_var: false,
             exclude_slash_tmp: false,