[codex] Make contextual user fragments dyn-renderable (#23397)

## Why
`ContextualUserFragment` needs to be usable behind `dyn` for render-only
paths, but associated constants made the trait non-object-safe.

## What changed
- Replaced associated constants with trait methods so `dyn
ContextualUserFragment` can render fragments.
- Preserved the existing typed `T::matches_text(text)` registration
pattern via `type_markers()`.
- Kept default `render()` on the main trait so implementations only
provide role, markers, and body.
- Added unit coverage for rendering a `Box<dyn ContextualUserFragment>`.

## Verification
- `cargo test -p codex-core contextual_user_fragment_is_dyn_compatible`
- `just fix -p codex-core`

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

@@ -12,9 +12,17 @@ impl PluginInstructions {
 }
 
 impl ContextualUserFragment for PluginInstructions {
-    const ROLE: &'static str = "developer";
-    const START_MARKER: &'static str = "";
-    const END_MARKER: &'static str = "";
+    fn role() -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        ("", "")
+    }
 
     fn body(&self) -> String {
         self.text.clone()