fix(agent): reset recovery chains across injections

Reset both the recovered segments and retry budget whenever injected input starts a new logical answer. Cover fatal tool-error boundaries and rename the prompt test module so pytest can collect the full suite.

tests/utils/test_length_recovery_runtime.py

@@ -0,0 +1,16 @@
+"""Tests for length-recovery prompt construction."""
+
+from nanobot.utils.runtime import build_length_recovery_message
+
+
+def test_length_recovery_message_anchors_the_existing_tail() -> None:
+    omitted_prefix = "OMITTED_PREFIX"
+    tail = "x" * 64
+
+    message = build_length_recovery_message(omitted_prefix + tail)
+
+    assert message["role"] == "user"
+    assert omitted_prefix not in message["content"]
+    assert f"<already_delivered_tail>\n{tail}\n</already_delivered_tail>" in message["content"]
+    assert "Output only new continuation text" in message["content"]
+    assert "Break remaining work into smaller steps" not in message["content"]