[stack 1/4] Split realtime websocket methods by version (#14828)

## Stack Position
1/4. Base PR in the realtime stack.

## Base
- `main`

## Unblocks
- #14830

## Scope
- Split the realtime websocket request builders into `common`, `v1`, and
`v2` modules.
- Keep runtime behavior unchanged in this PR.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/endpoint/realtime_websocket/mod.rs

@@ -1,4 +1,7 @@
 pub mod methods;
+mod methods_common;
+mod methods_v1;
+mod methods_v2;
 pub mod protocol;
 mod protocol_common;
 mod protocol_v1;