feat: add extension turn-input contributors (#25959)

## Disclaimer
Do not use for now

## Why

Extensions can already contribute prompt fragments and request same-turn
item injection, but there was no host-owned hook for contributing
structured `ResponseItem`s while Codex is assembling a new turn's
initial model input. This change adds that seam so extensions can attach
turn-local input that depends on the submitted user input and resolved
turn environments without routing through prompt text or late injection.

## What changed

- add `TurnInputContributor` to `codex_extension_api` and export the new
`TurnInputContext` / `TurnInputEnvironment` types it receives
- teach `ExtensionRegistry` to register and expose turn-input
contributors alongside the existing extension hooks
- call registered turn-input contributors from
`core/src/session/turn.rs` while building the initial injected input for
a turn, then append their returned `ResponseItem`s after the skill and
plugin injections

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

@@ -2,6 +2,7 @@ use std::future::Future;
 use std::sync::Arc;
 
 use codex_protocol::items::TurnItem;
+use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::ReviewDecision;
 use codex_protocol::protocol::TokenUsageInfo;
 use codex_tools::ToolCall;
@@ -12,6 +13,7 @@ use crate::ExtensionData;
 mod prompt;
 mod thread_lifecycle;
 mod tool_lifecycle;
+mod turn_input;
 mod turn_lifecycle;
 
 pub use prompt::PromptFragment;
@@ -25,6 +27,8 @@ pub use tool_lifecycle::ToolCallSource;
 pub use tool_lifecycle::ToolFinishInput;
 pub use tool_lifecycle::ToolLifecycleFuture;
 pub use tool_lifecycle::ToolStartInput;
+pub use turn_input::TurnInputContext;
+pub use turn_input::TurnInputEnvironment;
 pub use turn_lifecycle::TurnAbortInput;
 pub use turn_lifecycle::TurnErrorInput;
 pub use turn_lifecycle::TurnStartInput;
@@ -85,6 +89,25 @@ pub trait TurnLifecycleContributor: Send + Sync {
     async fn on_turn_error(&self, _input: TurnErrorInput<'_>) {}
 }
 
+/// WARNING: DO NOT USE YET
+/// Extension contribution that can add turn-local model input.
+///
+/// Implementations should resolve only the model-visible input they own and
+/// must preserve authority boundaries for external resources. Expensive or
+/// host-specific dependencies belong on the extension value installed by the
+/// host, not in this input.
+#[async_trait::async_trait]
+pub trait TurnInputContributor: Send + Sync {
+    /// Returns additional model input items for one submitted turn.
+    async fn contribute(
+        &self,
+        input: TurnInputContext,
+        session_store: &ExtensionData,
+        thread_store: &ExtensionData,
+        turn_store: &ExtensionData,
+    ) -> Vec<ResponseItem>;
+}
+
 /// Contributor for host-owned configuration changes.
 ///
 /// Implementations should treat the supplied values as immutable before/after