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.lock

@@ -1676,6 +1676,7 @@ dependencies = [
  "codex-config",
  "codex-core",
  "codex-exec",
+ "codex-exec-server",
  "codex-execpolicy",
  "codex-features",
  "codex-login",
@@ -1994,6 +1995,7 @@ dependencies = [
  "uuid",
  "walkdir",
  "which 8.0.0",
+ "whoami",
  "windows-sys 0.52.0",
  "wiremock",
  "zip 2.4.2",
@@ -11762,6 +11764,7 @@ checksum = "5d4a4db5077702ca3015d3d02d74974948aba2ad9e12ab7df718ee64ccd7e97d"
 dependencies = [
  "libredox",
  "wasite",
+ "web-sys",
 ]
 
 [[package]]