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/turn_input.rs

@@ -0,0 +1,25 @@
+use std::path::PathBuf;
+
+use codex_protocol::user_input::UserInput;
+
+/// Host-owned turn environment summary visible to turn-input contributors.
+#[derive(Debug, Clone)]
+pub struct TurnInputEnvironment {
+    /// Stable host environment id used to route executor-scoped capabilities.
+    pub environment_id: String,
+    /// Effective working directory for this turn in the environment.
+    pub cwd: PathBuf,
+    /// Whether this is the primary environment for the turn.
+    pub is_primary: bool,
+}
+
+/// Turn facts supplied before the host records turn-local model input items.
+#[derive(Debug, Clone)]
+pub struct TurnInputContext {
+    /// Stable host-owned turn identifier.
+    pub turn_id: String,
+    /// User input submitted for this turn.
+    pub user_input: Vec<UserInput>,
+    /// Resolved turn environments, in host priority order.
+    pub environments: Vec<TurnInputEnvironment>,
+}