refactor(agent): defer transcript assembly to runner (#5608)

* refactor(agent): defer transcript assembly to runner

Keep persisted history and the fresh turn as explicit inputs until the Runner assembles the provider transcript. Preserve ContextBuilder and direct AgentRunner compatibility while making the save boundary structural.

Refs NAN-81.

* fix(providers): preserve mixed adjacent user content

tests/agent/test_runner_core.py

@@ -10,6 +10,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 
 from agent.runner_helpers import make_run_spec
+from nanobot.agent.context import TranscriptInput
 from nanobot.config.schema import AgentDefaults
 from nanobot.providers.base import (
     LLMProvider,
@@ -34,6 +35,35 @@ def _make_usage_spec(provider, tools):
     )
 
 
+def test_initial_transcript_is_built_from_structured_turn_input() -> None:
+    from nanobot.agent.runner import AgentRunner
+
+    provider = MagicMock(spec=LLMProvider)
+    transcript_input = TranscriptInput(
+        history=[{"role": "user", "content": "earlier"}],
+        current_message="fresh",
+    )
+    expected = [
+        {"role": "system", "content": "system"},
+        {"role": "user", "content": "earlier"},
+        {"role": "user", "content": "fresh"},
+    ]
+    transcript_builder = MagicMock(return_value=expected)
+    spec = make_run_spec(
+        provider,
+        initial_messages=None,
+        transcript_input=transcript_input,
+        transcript_builder=transcript_builder,
+        tools=MagicMock(),
+        model="test-model",
+        max_iterations=1,
+        max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+    )
+
+    assert AgentRunner._initial_transcript(spec) == expected
+    transcript_builder.assert_called_once_with(transcript_input)
+
+
 def test_usage_or_estimate_replaces_reported_zero_for_content(monkeypatch) -> None:
     from nanobot.agent.runner import AgentRunner