[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/methods_v1.rs

@@ -0,0 +1,60 @@
+use crate::endpoint::realtime_websocket::methods_common::REALTIME_AUDIO_FORMAT;
+use crate::endpoint::realtime_websocket::methods_common::REALTIME_AUDIO_SAMPLE_RATE;
+use crate::endpoint::realtime_websocket::protocol::ConversationItemContent;
+use crate::endpoint::realtime_websocket::protocol::ConversationItemPayload;
+use crate::endpoint::realtime_websocket::protocol::ConversationMessageItem;
+use crate::endpoint::realtime_websocket::protocol::RealtimeOutboundMessage;
+use crate::endpoint::realtime_websocket::protocol::SessionAudio;
+use crate::endpoint::realtime_websocket::protocol::SessionAudioFormat;
+use crate::endpoint::realtime_websocket::protocol::SessionAudioInput;
+use crate::endpoint::realtime_websocket::protocol::SessionAudioOutput;
+use crate::endpoint::realtime_websocket::protocol::SessionAudioVoice;
+use crate::endpoint::realtime_websocket::protocol::SessionUpdateSession;
+
+const REALTIME_V1_SESSION_TYPE: &str = "quicksilver";
+
+pub(super) fn conversation_item_create_message(text: String) -> RealtimeOutboundMessage {
+    RealtimeOutboundMessage::ConversationItemCreate {
+        item: ConversationItemPayload::Message(ConversationMessageItem {
+            kind: "message".to_string(),
+            role: "user".to_string(),
+            content: vec![ConversationItemContent {
+                kind: "text".to_string(),
+                text,
+            }],
+        }),
+    }
+}
+
+pub(super) fn conversation_handoff_append_message(
+    handoff_id: String,
+    output_text: String,
+) -> RealtimeOutboundMessage {
+    RealtimeOutboundMessage::ConversationHandoffAppend {
+        handoff_id,
+        output_text,
+    }
+}
+
+pub(super) fn session_update_session(instructions: String) -> SessionUpdateSession {
+    SessionUpdateSession {
+        kind: REALTIME_V1_SESSION_TYPE.to_string(),
+        instructions: Some(instructions),
+        audio: SessionAudio {
+            input: SessionAudioInput {
+                format: SessionAudioFormat {
+                    kind: REALTIME_AUDIO_FORMAT.to_string(),
+                    rate: REALTIME_AUDIO_SAMPLE_RATE,
+                },
+            },
+            output: Some(SessionAudioOutput {
+                voice: SessionAudioVoice::Fathom,
+            }),
+        },
+        tools: None,
+    }
+}
+
+pub(super) fn websocket_intent() -> Option<&'static str> {
+    Some("quicksilver")
+}