refactor(chat): add streaming/agent/ package with build_main_agent_for_thread

Extracts the agent-construction wrapper that the chat streamers call to
materialize the LangGraph agent for a given thread. Centralizes how we
pass the agent factory plus checkpointer, runtime context, and the
in-memory content builder.

Add-only; pre-existing inline equivalent in stream_new_chat.py stays
until cutover.

surfsense_backend/app/tasks/chat/streaming/agent/__init__.py

@@ -0,0 +1,8 @@
+"""Agent construction and per-turn event-loop drivers."""
+
+from __future__ import annotations
+
+from app.tasks.chat.streaming.agent.builder import build_main_agent_for_thread
+from app.tasks.chat.streaming.agent.event_loop import stream_agent_events
+
+__all__ = ["build_main_agent_for_thread", "stream_agent_events"]

surfsense_backend/app/tasks/chat/streaming/agent/builder.py

@@ -0,0 +1,49 @@
+"""Single per-thread agent (re)build path.
+
+A graph swap mid-turn would corrupt checkpointer state for the same
+``thread_id``, so both the initial build and any mid-stream 429 recovery rebuild
+must funnel through this single function.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from app.agents.new_chat.filesystem_selection import FilesystemSelection
+from app.agents.new_chat.llm_config import AgentConfig
+from app.db import ChatVisibility
+from app.services.connector_service import ConnectorService
+
+
+async def build_main_agent_for_thread(
+    agent_factory: Any,
+    *,
+    llm: Any,
+    search_space_id: int,
+    db_session: Any,
+    connector_service: ConnectorService,
+    checkpointer: Any,
+    user_id: str | None,
+    thread_id: int | None,
+    agent_config: AgentConfig | None,
+    firecrawl_api_key: str | None,
+    thread_visibility: ChatVisibility | None,
+    filesystem_selection: FilesystemSelection | None,
+    disabled_tools: list[str] | None = None,
+    mentioned_document_ids: list[int] | None = None,
+) -> Any:
+    return await agent_factory(
+        llm=llm,
+        search_space_id=search_space_id,
+        db_session=db_session,
+        connector_service=connector_service,
+        checkpointer=checkpointer,
+        user_id=user_id,
+        thread_id=thread_id,
+        agent_config=agent_config,
+        firecrawl_api_key=firecrawl_api_key,
+        thread_visibility=thread_visibility,
+        filesystem_selection=filesystem_selection,
+        disabled_tools=disabled_tools,
+        mentioned_document_ids=mentioned_document_ids,
+    )