realtime: rename provider session ids (#20361)

## Summary

Codex is repurposing `session` to mean a thread group, so the realtime
provider session id should no longer use `session_id` / `sessionId` in
Codex-facing protocol payloads. This PR renames that provider-specific
field to `realtime_session_id` / `realtimeSessionId` and intentionally
breaks clients that still send the old field names.

## What Changed

- Renamed realtime provider session fields in `ConversationStartParams`,
`RealtimeConversationStartedEvent`, and `RealtimeEvent::SessionUpdated`.
- Renamed app-server v2 realtime request and notification fields to
`realtimeSessionId`.
- Removed legacy serde aliases for `session_id` / `sessionId`; clients
must send the new names.
- Propagated the rename through core realtime startup, app-server
adapters, codex-api websocket handling, and TUI realtime state.
- Regenerated app-server protocol schema/TypeScript outputs and updated
app-server README examples.
- Kept upstream Realtime API concepts unchanged: provider `session.id`
parsing and `x-session-id` headers still use the upstream wire names.

## Testing

- CI is running on the latest pushed commit.
- Earlier local verification on this PR:
  - `cargo test -p codex-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-core
realtime_conversation`
  - `cargo test -p codex-app-server-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-app-server
realtime_conversation`
- attempted `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-tui` (local
linker bus error while linking the test binary)

---------

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

codex-rs/app-server/src/codex_message_processor.rs

@@ -6994,7 +6994,7 @@ impl CodexMessageProcessor {
                 Op::RealtimeConversationStart(ConversationStartParams {
                     output_modality: params.output_modality,
                     prompt: params.prompt,
-                    session_id: params.session_id,
+                    realtime_session_id: params.realtime_session_id,
                     transport: params.transport.map(|transport| match transport {
                         ThreadRealtimeStartTransport::Websocket => {
                             ConversationStartTransport::Websocket