[codex] Make contextual user fragments dyn-renderable

[pakrym-oai]

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