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/BUILD.bazel

@@ -29,6 +29,7 @@ codex_rust_crate(
     },
     integration_compile_data_extra = [
         "//codex-rs/apply-patch:apply_patch_tool_instructions.md",
+        "templates/realtime/backend_prompt.md",
     ],
     integration_test_timeout = "long",
     test_data_extra = [