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/ext/extension-api/src/contributors/context.rs

@@ -0,0 +1,20 @@
+use codex_protocol::ThreadId;
+
+use crate::ExtensionData;
+
+/// Host context available while extensions contribute turn-scoped context fragments.
+#[derive(Clone, Copy)]
+pub struct TurnContextContributionInput<'a> {
+    /// Stable host-owned thread identifier.
+    pub thread_id: ThreadId,
+    /// Stable host-owned turn identifier.
+    pub turn_id: &'a str,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn.
+    pub turn_store: &'a ExtensionData,
+    /// Effective model context window for this turn, when known.
+    pub model_context_window: Option<i64>,
+}