refactor(agents): move error taxonomy to app/agents/shared/errors (slice 1)

First slice of promoting the shared agent toolkit out of the misnamed
`new_chat` package into the cross-agent `app/agents/shared` kernel.

`errors.py` is a leaf module (no intra-package deps) consumed by the
multi-agent chat, the chat streaming flows/monolith, and tests — i.e. it is
shared infrastructure, not single-agent code. Moved it verbatim to
`app.agents.shared.errors` and flipped all 12 importers. No re-export shim
remains since zero importers needed it.

Behavior-preserving: identical class/enum definitions; only the import path
changes. 1208 agent + chat-task tests green.

surfsense_backend/tests/unit/tasks/chat/streaming/test_stage_1_parity.py

@@ -16,7 +16,7 @@ from typing import Any
 
 import pytest
 
-from app.agents.new_chat.errors import BusyError
+from app.agents.shared.errors import BusyError
 from app.agents.new_chat.middleware.busy_mutex import request_cancel, reset_cancel
 from app.tasks.chat.stream_new_chat import (
     _classify_stream_exception as old_classify,