Use static instructions for the Default collaboration mode (#42904)

## What changed

Write Default and Plan directly into the default mode instructions and use the text without template rendering. Remove the mode-name formatting helpers and the `codex-utils-template` dependency from `codex-models-manager`.

Update the instruction test to retain checks for `request_user_input` availability guidance while removing placeholder assertions.

GitOrigin-RevId: d6afa478c30f991baa9ef4d5459e84354f29ce17

codex-rs/models-manager/Cargo.toml

@@ -20,7 +20,6 @@ codex-login = { workspace = true }
 codex-otel = { workspace = true }
 codex-protocol = { workspace = true }
 codex-utils-output-truncation = { workspace = true }
-codex-utils-template = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 tokio = { workspace = true, features = ["fs", "sync", "time"] }