Add per-account approval settings for apps (#42047)

## What changed

- Add `apps.<app_id>.links.<link_id>` configuration for overriding
  `approvals_reviewer` and `default_tools_approval_mode` per connected account.
- Expose link settings through the app-server protocol and generated JSON and
  TypeScript schemas.

## Testing

- Extend `config/read` coverage for populated, empty, and absent `links`
  sections.

GitOrigin-RevId: 43004d2722439060fdadc5e5cd9ed3108774ecaf

codex-rs/core/src/mcp_tool_call_tests.rs

@@ -2065,6 +2065,7 @@ async fn persist_codex_app_tool_approval_writes_tool_override() {
                             },
                         )]),
                     }),
+                    links: None,
                 },
             )]),
         })