feat: introduce Op:UserTurn (#2329)

This introduces `Op::UserTurn`, which makes it possible to override many
of the fields that were set when the `Session` was originally created
when creating a new conversation turn. This is one way we could support
changing things like `model` or `cwd` in the middle of the conversation,
though we may want to consider making each field optional, or
alternatively having a separate `Op` that mutates the `TurnContext`
associated with a `submission_loop()`.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2329).
* #2345
* __->__ #2329
* #2343
* #2340
* #2338

codex-rs/core/src/protocol.rs

@@ -17,6 +17,8 @@ use serde_bytes::ByteBuf;
 use strum_macros::Display;
 use uuid::Uuid;
 
+use crate::config_types::ReasoningEffort as ReasoningEffortConfig;
+use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::message_history::HistoryEntry;
 use crate::parse_command::ParsedCommand;
 use crate::plan_tool::UpdatePlanArgs;
@@ -46,6 +48,33 @@ pub enum Op {
         items: Vec<InputItem>,
     },
 
+    /// Similar to [`Op::UserInput`], but contains additional context required
+    /// for a turn of a [`crate::codex_conversation::CodexConversation`].
+    UserTurn {
+        /// User input items, see `InputItem`
+        items: Vec<InputItem>,
+
+        /// `cwd` to use with the [`SandboxPolicy`] and potentially tool calls
+        /// such as `local_shell`.
+        cwd: PathBuf,
+
+        /// Policy to use for command approval.
+        approval_policy: AskForApproval,
+
+        /// Policy to use for tool calls such as `local_shell`.
+        sandbox_policy: SandboxPolicy,
+
+        /// Must be a valid model slug for the [`crate::client::ModelClient`]
+        /// associated with this conversation.
+        model: String,
+
+        /// Will only be honored if the model is configured to use reasoning.
+        effort: ReasoningEffortConfig,
+
+        /// Will only be honored if the model is configured to use reasoning.
+        summary: ReasoningSummaryConfig,
+    },
+
     /// Approve a command execution
     ExecApproval {
         /// The id of the submission we are approving