test: pin per-turn reasoning extraction semantics

Covers four scenarios for the reasoning-box extraction loop:
 - simple turn with reasoning
 - simple turn with no reasoning
 - tool-calling turn where reasoning lives on the tool-call step
 - prior turn had reasoning, current turn does not (the stale-display
   bug the fix exists for)
 - tool-calling turn where reasoning lives on BOTH steps (latest wins)
 - empty-string reasoning treated as missing

Also updates the four inline replica loops in tests/cli/test_reasoning_command.py
to match the new turn-boundary shape so the test file reflects
production semantics.

tests/run_agent/test_last_reasoning_per_turn.py

@@ -0,0 +1,107 @@
+"""Tests for per-turn reasoning extraction in AIAgent.run_conversation.
+
+Verifies the reasoning field returned to display layers (CLI reasoning box,
+gateway reasoning footer, TUI reasoning event) only reflects the CURRENT
+turn's reasoning — never leaks from a prior turn — and is picked up
+correctly when reasoning is attached to a tool-calling assistant step
+rather than the final-answer assistant step.
+"""
+from __future__ import annotations
+
+
+def _extract_last_reasoning(messages):
+    """Replica of the extraction loop in run_agent.py (~line 13867).
+
+    Tests pin the loop's behaviour so that refactors can't silently
+    regress the per-turn semantic.
+    """
+    last_reasoning = None
+    for msg in reversed(messages):
+        if msg.get("role") == "user":
+            break
+        if msg.get("role") == "assistant" and msg.get("reasoning"):
+            last_reasoning = msg["reasoning"]
+            break
+    return last_reasoning
+
+
+def test_simple_turn_reasoning_present():
+    messages = [
+        {"role": "user", "content": "hello"},
+        {"role": "assistant", "content": "hi", "reasoning": "greeting the user"},
+    ]
+    assert _extract_last_reasoning(messages) == "greeting the user"
+
+
+def test_simple_turn_no_reasoning():
+    messages = [
+        {"role": "user", "content": "hello"},
+        {"role": "assistant", "content": "hi", "reasoning": None},
+    ]
+    assert _extract_last_reasoning(messages) is None
+
+
+def test_tool_call_turn_reasoning_on_tool_call_step():
+    """When the model reasons on the tool-call step and the final-answer
+    step has no reasoning (Claude thinking / DeepSeek v4 / Codex Responses
+    pattern), the box must show the tool-call-step reasoning, not empty.
+    """
+    messages = [
+        {"role": "user", "content": "search the repo for X"},
+        {
+            "role": "assistant",
+            "content": "",
+            "reasoning": "I should use search_files",
+            "tool_calls": [{"id": "c1", "type": "function",
+                            "function": {"name": "search_files", "arguments": "{}"}}],
+        },
+        {"role": "tool", "tool_call_id": "c1", "content": "3 matches"},
+        {"role": "assistant", "content": "Found 3 matches", "reasoning": None},
+    ]
+    assert _extract_last_reasoning(messages) == "I should use search_files"
+
+
+def test_no_stale_reasoning_across_turns():
+    """The regression the whole change exists for.  Prior turn had
+    reasoning; current turn has none.  The reasoning box must NOT show
+    the prior turn's text.
+    """
+    messages = [
+        # prior turn
+        {"role": "user", "content": "explain quantum tunneling"},
+        {"role": "assistant", "content": "It's when...",
+         "reasoning": "tunneling happens when particles..."},
+        # current turn
+        {"role": "user", "content": "thanks"},
+        {"role": "assistant", "content": "You're welcome!", "reasoning": None},
+    ]
+    assert _extract_last_reasoning(messages) is None
+
+
+def test_tool_call_turn_picks_latest_reasoning_within_turn():
+    """If BOTH the tool-call step and the final step have reasoning
+    (uncommon but possible), the final-step reasoning wins — it's the
+    most recent thought within the current turn.
+    """
+    messages = [
+        {"role": "user", "content": "search and summarize"},
+        {
+            "role": "assistant",
+            "content": "",
+            "reasoning": "initial plan",
+            "tool_calls": [{"id": "c1", "type": "function",
+                            "function": {"name": "search_files", "arguments": "{}"}}],
+        },
+        {"role": "tool", "tool_call_id": "c1", "content": "results"},
+        {"role": "assistant", "content": "Here's the summary",
+         "reasoning": "synthesized view of results"},
+    ]
+    assert _extract_last_reasoning(messages) == "synthesized view of results"
+
+
+def test_empty_string_reasoning_treated_as_missing():
+    messages = [
+        {"role": "user", "content": "hi"},
+        {"role": "assistant", "content": "hello", "reasoning": ""},
+    ]
+    assert _extract_last_reasoning(messages) is None