[codex] Support assistant realtime append text (#28836)

## Why

Frontend realtime voice continuity needs to replay a tiny
previous-session overlap as actual conversation items, including
assistant text. The app-server `thread/realtime/appendText` API already
carries a role through to the Rust realtime websocket layer, but the
shared role enum only accepted `user` and `developer`.

## What Changed

- Added `assistant` to `ConversationTextRole` and regenerated the
app-server schema/type fixtures.
- Added `output_text` as a realtime conversation content type.
- Updated realtime websocket item creation so assistant appendText emits
`content: [{ type: "output_text", text }]`, while user and developer
continue to emit `input_text`.
- Updated app-server docs and tests to cover assistant appendText
alongside the existing developer role behavior.

## Validation

- `just write-app-server-schema`
- `just fmt` (first sandboxed attempt failed because `uv` could not
access `~/.cache/uv`; reran with filesystem access and passed)
- `just test -p codex-api` passed: 126/126
- `just test -p codex-app-server-protocol` passed: 239/239, including
generated JSON/TypeScript fixture checks
- `just test -p codex-app-server` was started locally but stopped per
request after unrelated local sandbox/Seatbelt failures (`sandbox-exec:
sandbox_apply: Operation not permitted`) and one missing local `codex`
binary failure; CI should be faster and more authoritative for the full
suite.

codex-rs/protocol/src/protocol.rs

@@ -417,6 +417,7 @@ pub enum ConversationTextRole {
     #[default]
     User,
     Developer,
+    Assistant,
 }
 
 #[derive(Debug, Clone, PartialEq)]