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/core/Cargo.toml

@@ -116,6 +116,7 @@ tracing = { workspace = true, features = ["log"] }
 url = { workspace = true }
 uuid = { workspace = true, features = ["serde", "v4", "v5"] }
 which = { workspace = true }
+whoami = { workspace = true }
 zip = { workspace = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]