fix(agent): default request concurrency to unlimited (#5572)

* fix(agent): default request concurrency to unlimited

* test(agent): clarify session serialization coverage

tests/agent/test_loop_concurrency.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+import pytest
+
+
+def _provider() -> MagicMock:
+    provider = MagicMock()
+    provider.get_default_model.return_value = "test-model"
+    provider.generation = SimpleNamespace(
+        max_tokens=4096,
+        temperature=0.1,
+        reasoning_effort=None,
+    )
+    return provider
+
+
+def test_request_concurrency_is_unlimited_by_default(
+    monkeypatch: pytest.MonkeyPatch,
+    loop_factory,
+) -> None:
+    monkeypatch.delenv("NANOBOT_MAX_CONCURRENT_REQUESTS", raising=False)
+
+    loop = loop_factory(provider=_provider(), patch_deps=True)
+
+    assert loop._concurrency_gate is None
+
+
+@pytest.mark.asyncio
+async def test_positive_request_concurrency_keeps_explicit_cap(
+    monkeypatch: pytest.MonkeyPatch,
+    loop_factory,
+) -> None:
+    monkeypatch.setenv("NANOBOT_MAX_CONCURRENT_REQUESTS", "2")
+    loop = loop_factory(provider=_provider(), patch_deps=True)
+    gate = loop._concurrency_gate
+
+    assert gate is not None
+    for _ in range(2):
+        await gate.acquire()
+    try:
+        assert gate.locked()
+    finally:
+        for _ in range(2):
+            gate.release()