refactor(agents): move tools package to app/agents/shared (slice 6)

Relocate the entire new_chat/tools/ package (62 files incl. registry, hitl, MCP
cluster, and all connector subpackages: gmail/slack/discord/teams/drive/etc.)
to the shared kernel. The package turned out to be a clean cohesive cluster:
its only references to non-tools new_chat modules were comments, and its
middleware deps were already flipped to shared in slice 5c.

Flip 33 live importers (multi-agent, flows, routes, services, anonymous_agent,
tests). Re-export shims remain for the frozen single-agent stack: a package
__init__ mirroring the public surface (new_chat.__init__ imports it) plus
invalid_tool + registry submodule shims (chat_deepagent imports those).

Resolves slice 5c's two transient back-edges: shared/middleware/action_log
(TYPE_CHECKING ToolDefinition) and tool_call_repair (local INVALID_TOOL_NAME)
now point at app.agents.shared.tools.

surfsense_backend/app/agents/shared/tools/invalid_tool.py

@@ -0,0 +1,53 @@
+"""
+The ``invalid`` fallback tool.
+
+When the model emits a tool call whose name doesn't match any registered
+tool, :class:`ToolCallNameRepairMiddleware` rewrites the call to ``invalid``
+with the original name and a parser/validation error string. This tool's
+execution then returns that error to the model so it can self-correct.
+
+Ported from OpenCode's ``packages/opencode/src/tool/invalid.ts`` —
+LangChain has no equivalent fallback path; the default behavior on an
+unknown tool name is a hard ``ToolNotFoundError`` which kills the turn.
+
+Critically, the :class:`ToolDefinition` for this tool is **excluded** from
+the system-prompt tool list and from ``LLMToolSelectorMiddleware`` selection
+(see ``ToolDefinition.always_include`` filtering in the registry) — the
+model never advertises ``invalid`` as a callable. It only ever shows up
+in the tool registry so LangGraph can dispatch the rewritten call.
+"""
+
+from __future__ import annotations
+
+from langchain_core.tools import tool
+
+INVALID_TOOL_NAME = "invalid"
+INVALID_TOOL_DESCRIPTION = "Do not use"
+
+
+def _format_invalid_message(tool: str | None, error: str | None) -> str:
+    """Return the user-visible error string. Mirrors ``invalid.ts``."""
+    name = tool or "<unknown>"
+    detail = error or "(no error message provided)"
+    return (
+        f"The arguments provided to the tool `{name}` are invalid: {detail}\n"
+        f"Read the tool's docstring carefully and try again with valid arguments."
+    )
+
+
+@tool(name_or_callable=INVALID_TOOL_NAME, description=INVALID_TOOL_DESCRIPTION)
+def invalid_tool(tool: str | None = None, error: str | None = None) -> str:
+    """Return a human-readable explanation of a tool-call validation failure.
+
+    Activated only when :class:`ToolCallNameRepairMiddleware` rewrites a
+    failed tool call to ``invalid`` with the original tool name and the
+    error message produced during validation.
+    """
+    return _format_invalid_message(tool, error)
+
+
+__all__ = [
+    "INVALID_TOOL_DESCRIPTION",
+    "INVALID_TOOL_NAME",
+    "invalid_tool",
+]