Split DeveloperInstructions into individual fragments. (#18813)

Split DeveloperInstructions into individual fragments.

codex-rs/core/src/context/plugin_instructions.rs

@@ -0,0 +1,22 @@
+use super::ContextualUserFragment;
+
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct PluginInstructions {
+    text: String,
+}
+
+impl PluginInstructions {
+    pub(crate) fn new(text: impl Into<String>) -> Self {
+        Self { text: text.into() }
+    }
+}
+
+impl ContextualUserFragment for PluginInstructions {
+    const ROLE: &'static str = "developer";
+    const START_MARKER: &'static str = "";
+    const END_MARKER: &'static str = "";
+
+    fn body(&self) -> String {
+        self.text.clone()
+    }
+}