Add tools for workspace resource

[mathislucka]

Overview

Look at REPO.md for general instructions on this task.

We want tools to interact with the workspace resource.

The tools we want are:

• list_workspaces
• get_workspace
• create_workspace

Create the tools in src/deepeset_mcp/tools/ following the existing patterns.

Register the tools in src/deepset_mcp/tool_factory.py. All the tools should have memory type "explorable".

Add unit tests for the tools using a Fake Resource. Follow the example of other tool tests.