app-server: use profile ids in v2 permission params (#23360)

## Why

The v2 app-server permission profile fields are experimental, but the
previous migration kept a legacy object payload for profile selection.
That made clients aware of server-owned `activePermissionProfile`
metadata such as `extends`, and it kept a
`legacy_additional_writable_roots` path even though
`runtimeWorkspaceRoots` now owns runtime workspace-root selection.

This PR makes the client contract match the intended model: clients
select a permission profile by id, and the server resolves and reports
active profile provenance in response payloads.

Follow-up to #22611.

## What Changed

- Changed `thread/start`, `thread/resume`, `thread/fork`, and
`turn/start` permission profile selection to plain profile id strings.
- Changed `command/exec.permissionProfile` to a plain profile id string
for the same client/server ownership split.
- Removed `PermissionProfileSelectionParams` and the legacy `{ type:
"profile", modifications: [...] }` compatibility deserializer.
- Updated app-server, TUI, and `codex exec` call sites to send only ids,
while keeping `activePermissionProfile` as server response metadata.
- Updated app-server docs and schema fixtures for the revised
`command/exec.permissionProfile` shape.

## Verification

- `cargo test -p codex-app-server-protocol`
- `RUST_MIN_STACK=8388608 cargo test -p codex-app-server`
- `cargo test -p codex-exec`
- `RUST_MIN_STACK=8388608 cargo test -p codex-tui`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/23360).
* #23368
* __->__ #23360

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1555,6 +1555,7 @@ mod tests {
     use anyhow::Result;
     use codex_protocol::ThreadId;
     use codex_protocol::account::PlanType;
+    use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_READ_ONLY;
     use codex_protocol::parse_command::ParsedCommand;
     use codex_protocol::protocol::RealtimeConversationVersion;
     use codex_protocol::protocol::RealtimeOutputModality;
@@ -2994,7 +2995,7 @@ mod tests {
                 env: None,
                 size: None,
                 sandbox_policy: None,
-                permission_profile: Some(v2::ActivePermissionProfile::read_only()),
+                permission_profile: Some(BUILT_IN_PERMISSION_PROFILE_READ_ONLY.to_string()),
             },
         };