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.rs

@@ -11,6 +11,7 @@ use codex_tools::ToolExecutor;
 
 use crate::ExtensionData;
 
+mod context;
 mod mcp;
 mod prompt;
 mod thread_lifecycle;
@@ -18,6 +19,7 @@ mod tool_lifecycle;
 mod turn_input;
 mod turn_lifecycle;
 
+pub use context::TurnContextContributionInput;
 pub use mcp::McpServerContribution;
 pub use mcp::McpServerContributionContext;
 pub use prompt::PromptFragment;
@@ -62,12 +64,34 @@ pub trait McpServerContributor<C: Sync>: Send + Sync {
 }
 
 /// Extension contribution that adds prompt fragments during prompt assembly.
+///
+/// Implementations should use the method matching the scope needed by the
+/// fragment: thread/session context for stable inputs, and turn context for
+/// fragments that depend on turn-local host state.
 pub trait ContextContributor: Send + Sync {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         session_store: &'a ExtensionData,
         thread_store: &'a ExtensionData,
-    ) -> ExtensionFuture<'a, Vec<PromptFragment>>;
+    ) -> ExtensionFuture<'a, Vec<PromptFragment>> {
+        Box::pin(async move {
+            let _self = self;
+            let _session_store = session_store;
+            let _thread_store = thread_store;
+            Vec::new()
+        })
+    }
+
+    fn contribute_turn_context<'a>(
+        &'a self,
+        input: TurnContextContributionInput<'a>,
+    ) -> ExtensionFuture<'a, Vec<PromptFragment>> {
+        Box::pin(async move {
+            let _self = self;
+            let _input = input;
+            Vec::new()
+        })
+    }
 }
 
 /// Contributor for host-owned thread lifecycle gates.

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>,
+}

codex-rs/ext/extension-api/src/lib.rs

@@ -54,6 +54,7 @@ pub use contributors::ToolLifecycleContributor;
 pub use contributors::ToolLifecycleFuture;
 pub use contributors::ToolStartInput;
 pub use contributors::TurnAbortInput;
+pub use contributors::TurnContextContributionInput;
 pub use contributors::TurnErrorInput;
 pub use contributors::TurnInputContext;
 pub use contributors::TurnInputContributor;