Add turn-scoped context contributions (#28911)

## Summary
- keep context injection on a single ContextContributor trait
- split context injection into thread-scoped and turn-scoped
contribution methods
- wire turn-scoped fragments into initial context assembly so extensions
can contribute context from turn-local state

codex-rs/protocol/src/protocol.rs

@@ -2992,7 +2992,7 @@ pub struct TurnContextNetworkItem {
 /// context updates, and again after mid-turn compaction when replacement
 /// history re-establishes full context, so resume/fork replay can recover the
 /// latest durable baseline.
-#[derive(Serialize, Deserialize, Clone, Debug, JsonSchema, TS)]
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema, TS)]
 pub struct TurnContextItem {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub turn_id: Option<String>,