test(agent): expand coverage and refactor test structure

- Add 42 tests for ContextBuilder (context.py: 0→42 tests)
- Add 37 tests for SubagentManager lifecycle (subagent.py: 2→37 tests)
- Add 42 unit tests for AutoCompact in isolation
- Split monolithic test_runner.py (3313 lines) into 9 focused files:
  test_runner_core, test_runner_hooks, test_runner_errors,
  test_runner_safety, test_runner_persistence, test_runner_governance,
  test_runner_tool_execution, test_runner_injections,
  test_loop_runner_integration
- Add 3 config passthrough tests (temperature/max_tokens/reasoning_effort)
- Fix fragile patch.object(__init__) in test_stop_preserves_context
- Create shared conftest.py with make_provider/make_loop factories

Total: 934 tests passing, 0 regressions

tests/agent/test_runner_tool_execution.py

@@ -0,0 +1,181 @@
+"""Tests for AgentRunner tool execution: batching, concurrency, exclusive tools."""
+
+from __future__ import annotations
+
+import asyncio
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from nanobot.agent.tools.base import Tool
+from nanobot.agent.tools.registry import ToolRegistry
+from nanobot.config.schema import AgentDefaults
+from nanobot.providers.base import LLMResponse, ToolCallRequest
+
+_MAX_TOOL_RESULT_CHARS = AgentDefaults().max_tool_result_chars
+
+class _DelayTool(Tool):
+    def __init__(
+        self,
+        name: str,
+        *,
+        delay: float,
+        read_only: bool,
+        shared_events: list[str],
+        exclusive: bool = False,
+    ):
+        self._name = name
+        self._delay = delay
+        self._read_only = read_only
+        self._shared_events = shared_events
+        self._exclusive = exclusive
+
+    @property
+    def name(self) -> str:
+        return self._name
+
+    @property
+    def description(self) -> str:
+        return self._name
+
+    @property
+    def parameters(self) -> dict:
+        return {"type": "object", "properties": {}, "required": []}
+
+    @property
+    def read_only(self) -> bool:
+        return self._read_only
+
+    @property
+    def exclusive(self) -> bool:
+        return self._exclusive
+
+    async def execute(self, **kwargs):
+        self._shared_events.append(f"start:{self._name}")
+        await asyncio.sleep(self._delay)
+        self._shared_events.append(f"end:{self._name}")
+        return self._name
+
+
+@pytest.mark.asyncio
+async def test_runner_batches_read_only_tools_before_exclusive_work():
+    from nanobot.agent.runner import AgentRunSpec, AgentRunner
+
+    tools = ToolRegistry()
+    shared_events: list[str] = []
+    read_a = _DelayTool("read_a", delay=0.05, read_only=True, shared_events=shared_events)
+    read_b = _DelayTool("read_b", delay=0.05, read_only=True, shared_events=shared_events)
+    write_a = _DelayTool("write_a", delay=0.01, read_only=False, shared_events=shared_events)
+    tools.register(read_a)
+    tools.register(read_b)
+    tools.register(write_a)
+
+    runner = AgentRunner(MagicMock())
+    await runner._execute_tools(
+        AgentRunSpec(
+            initial_messages=[],
+            tools=tools,
+            model="test-model",
+            max_iterations=1,
+            max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+            concurrent_tools=True,
+        ),
+        [
+            ToolCallRequest(id="ro1", name="read_a", arguments={}),
+            ToolCallRequest(id="ro2", name="read_b", arguments={}),
+            ToolCallRequest(id="rw1", name="write_a", arguments={}),
+        ],
+        {},
+        {},
+    )
+
+    assert shared_events[0:2] == ["start:read_a", "start:read_b"]
+    assert "end:read_a" in shared_events and "end:read_b" in shared_events
+    assert shared_events.index("end:read_a") < shared_events.index("start:write_a")
+    assert shared_events.index("end:read_b") < shared_events.index("start:write_a")
+    assert shared_events[-2:] == ["start:write_a", "end:write_a"]
+
+
+@pytest.mark.asyncio
+async def test_runner_does_not_batch_exclusive_read_only_tools():
+    from nanobot.agent.runner import AgentRunSpec, AgentRunner
+
+    tools = ToolRegistry()
+    shared_events: list[str] = []
+    read_a = _DelayTool("read_a", delay=0.03, read_only=True, shared_events=shared_events)
+    read_b = _DelayTool("read_b", delay=0.03, read_only=True, shared_events=shared_events)
+    ddg_like = _DelayTool(
+        "ddg_like",
+        delay=0.01,
+        read_only=True,
+        shared_events=shared_events,
+        exclusive=True,
+    )
+    tools.register(read_a)
+    tools.register(ddg_like)
+    tools.register(read_b)
+
+    runner = AgentRunner(MagicMock())
+    await runner._execute_tools(
+        AgentRunSpec(
+            initial_messages=[],
+            tools=tools,
+            model="test-model",
+            max_iterations=1,
+            max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+            concurrent_tools=True,
+        ),
+        [
+            ToolCallRequest(id="ro1", name="read_a", arguments={}),
+            ToolCallRequest(id="ddg1", name="ddg_like", arguments={}),
+            ToolCallRequest(id="ro2", name="read_b", arguments={}),
+        ],
+        {},
+        {},
+    )
+
+    assert shared_events[0] == "start:read_a"
+    assert shared_events.index("end:read_a") < shared_events.index("start:ddg_like")
+    assert shared_events.index("end:ddg_like") < shared_events.index("start:read_b")
+
+
+@pytest.mark.asyncio
+async def test_runner_blocks_repeated_external_fetches():
+    from nanobot.agent.runner import AgentRunSpec, AgentRunner
+
+    provider = MagicMock()
+    captured_final_call: list[dict] = []
+    call_count = {"n": 0}
+
+    async def chat_with_retry(*, messages, **kwargs):
+        call_count["n"] += 1
+        if call_count["n"] <= 3:
+            return LLMResponse(
+                content="working",
+                tool_calls=[ToolCallRequest(id=f"call_{call_count['n']}", name="web_fetch", arguments={"url": "https://example.com"})],
+                usage={},
+            )
+        captured_final_call[:] = messages
+        return LLMResponse(content="done", tool_calls=[], usage={})
+
+    provider.chat_with_retry = chat_with_retry
+    tools = MagicMock()
+    tools.get_definitions.return_value = []
+    tools.execute = AsyncMock(return_value="page content")
+
+    runner = AgentRunner(provider)
+    result = await runner.run(AgentRunSpec(
+        initial_messages=[{"role": "user", "content": "research task"}],
+        tools=tools,
+        model="test-model",
+        max_iterations=4,
+        max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+    ))
+
+    assert result.final_content == "done"
+    assert tools.execute.await_count == 2
+    blocked_tool_message = [
+        msg for msg in captured_final_call
+        if msg.get("role") == "tool" and msg.get("tool_call_id") == "call_3"
+    ][0]
+    assert "repeated external lookup blocked" in blocked_tool_message["content"]