Move default realtime prompt into core (#17165)

- Adds a core-owned realtime backend prompt template and preparation
path.
- Makes omitted realtime start prompts use the core default, while null
or empty prompts intentionally send empty instructions.
- Covers the core realtime path and app-server v2 path with integration
coverage.

---------

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

codex-rs/Cargo.toml

@@ -352,6 +352,7 @@ vt100 = "0.16.2"
 walkdir = "2.5.0"
 webbrowser = "1.0"
 which = "8"
+whoami = "1.6.1"
 wildmatch = "2.6.1"
 zip = "2.4.2"
 zstd = "0.13"