Parse reasoning text content (#2277)

Sometimes COT is returns as text content instead of `ReasoningText`. We
should parse it but not serialize back on requests.

---------

Co-authored-by: Ahmed Ibrahim <aibrahim@openai.com>

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__deltas_then_same_final_message_are_rendered_snapshot.snap

@@ -0,0 +1,10 @@
+---
+source: tui/src/chatwidget/tests.rs
+assertion_line: 886
+expression: combined
+---
+thinking
+I will first analyze the request.
+
+codex
+Here is the result.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__final_reasoning_then_message_without_deltas_are_rendered.snap

@@ -0,0 +1,9 @@
+---
+source: tui/src/chatwidget/tests.rs
+expression: combined
+---
+thinking
+I will first analyze the request.
+
+codex
+Here is the result.

codex-rs/tui/src/chatwidget/tests.rs

@@ -12,6 +12,7 @@ use codex_core::plan_tool::UpdatePlanArgs;
 use codex_core::protocol::AgentMessageDeltaEvent;
 use codex_core::protocol::AgentMessageEvent;
 use codex_core::protocol::AgentReasoningDeltaEvent;
+use codex_core::protocol::AgentReasoningEvent;
 use codex_core::protocol::ApplyPatchApprovalRequestEvent;
 use codex_core::protocol::Event;
 use codex_core::protocol::EventMsg;
@@ -24,6 +25,7 @@ use codex_core::protocol::TaskCompleteEvent;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
 use crossterm::event::KeyModifiers;
+use insta::assert_snapshot;
 use pretty_assertions::assert_eq;
 use std::fs::File;
 use std::io::BufRead;
@@ -413,46 +415,6 @@ async fn binary_size_transcript_matches_ideal_fixture() {
     assert_eq!(visible_after, ideal);
 }
 
-#[test]
-fn final_longer_answer_after_single_char_delta_is_complete() {
-    let (mut chat, rx, _op_rx) = make_chatwidget_manual();
-
-    // Simulate a stray delta without newline (e.g., punctuation).
-    chat.handle_codex_event(Event {
-        id: "sub-x".into(),
-        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent { delta: "?".into() }),
-    });
-
-    // Now send the full final answer with no newline.
-    let full = "Hi! How can I help with codex-rs today? Want me to explore the repo, run tests, or work on a specific change?";
-    chat.handle_codex_event(Event {
-        id: "sub-x".into(),
-        msg: EventMsg::AgentMessage(AgentMessageEvent {
-            message: full.into(),
-        }),
-    });
-
-    // Drain and assert the full message appears in history.
-    let cells = drain_insert_history(&rx);
-    let mut found = false;
-    for lines in &cells {
-        let s = lines
-            .iter()
-            .flat_map(|l| l.spans.iter())
-            .map(|sp| sp.content.clone())
-            .collect::<String>();
-        if s.contains(full) {
-            found = true;
-            break;
-        }
-    }
-    assert!(
-        found,
-        "expected full final message to be flushed to history, cells={:?}",
-        cells.len()
-    );
-}
-
 #[test]
 fn apply_patch_events_emit_history_cells() {
     let (mut chat, rx, _op_rx) = make_chatwidget_manual();
@@ -923,3 +885,91 @@ fn multiple_agent_messages_in_single_turn_emit_multiple_headers() {
     let second_idx = combined.find("Second message").unwrap();
     assert!(first_idx < second_idx, "messages out of order: {combined}");
 }
+
+#[test]
+fn final_reasoning_then_message_without_deltas_are_rendered() {
+    let (mut chat, rx, _op_rx) = make_chatwidget_manual();
+
+    // No deltas; only final reasoning followed by final message.
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoning(AgentReasoningEvent {
+            text: "I will first analyze the request.".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessage(AgentMessageEvent {
+            message: "Here is the result.".into(),
+        }),
+    });
+
+    // Drain history and snapshot the combined visible content.
+    let cells = drain_insert_history(&rx);
+    let combined = cells
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<String>();
+    assert_snapshot!(combined);
+}
+
+#[test]
+fn deltas_then_same_final_message_are_rendered_snapshot() {
+    let (mut chat, rx, _op_rx) = make_chatwidget_manual();
+
+    // Stream some reasoning deltas first.
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "I will ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "first analyze the ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "request.".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoning(AgentReasoningEvent {
+            text: "request.".into(),
+        }),
+    });
+
+    // Then stream answer deltas, followed by the exact same final message.
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
+            delta: "Here is the ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
+            delta: "result.".into(),
+        }),
+    });
+
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessage(AgentMessageEvent {
+            message: "Here is the result.".into(),
+        }),
+    });
+
+    // Snapshot the combined visible content to ensure we render as expected
+    // when deltas are followed by the identical final message.
+    let cells = drain_insert_history(&rx);
+    let combined = cells
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<String>();
+    assert_snapshot!(combined);
+}