Reverts ArcadeAI/arcade-mcp#759
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> **Medium Risk**
> Adds a new LangChain/LangGraph integration layer that wraps and
executes Arcade tools, including authorization and error/interrupt
handling; bugs here could impact tool execution semantics for adopters.
Changes are mostly additive and scoped to `contrib/langchain`.
>
> **Overview**
> Re-introduces a standalone `contrib/langchain` Python package
(`langchain-arcade`) to expose Arcade tools as LangChain
`StructuredTool`s.
>
> Adds sync/async `ToolManager` implementations plus utilities to
generate Pydantic arg schemas from `ToolDefinition`, optionally rewrite
tool names (underscores vs dots), and handle authorization via LangGraph
`NodeInterrupt` or structured error responses.
>
> Includes packaging/dev scaffolding (`pyproject.toml`, `tox.ini`,
`Makefile`, `.gitignore`, `LICENSE`, `README`) and a comprehensive test
suite covering manager behaviors and auth flows.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
abd23b6d954470cb1e7376158468c0e59cdc7d7a. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
# PR Description
`arcade.toml` was deprecated in favor of `credentials.yaml` in PR #116 .
This PR completely removes any references to handling the deprecation &
any references to `arcade.toml`
