Model context protocol prompt fix and roots

[alexsmirnov]

Description

[ What changed? Feel free to be brief. ]
The fix updates prompt ( slash command ) obtained from MCP to properly include prompt context into model call.
Without it, all @... context resources stripped out from user message. The code based on /review and custom prompt command code. Noisy comments reflect my investigation into how it works. To confirm correctness, it also includes unit test that repeated all combinations that I collected from debug sessions.

The second change is support for MCP roots feature https://modelcontextprotocol.io/docs/concepts/roots.
Client reports workspace directories as response to request from server. I found it necessary to provide workspace specific resources and tools.
I do not dare to include my project https://github.com/alexsmirnov/mcp-server-continue as reference to documentation, it's too early stage. But it supports all features.

Checklist

• [] The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screenshots

[ For visual changes, include screenshots. ]

Testing instructions

with https://github.com/alexsmirnov/mcp-server-continue and uv https://github.com/astral-sh/uv
config.json:

{
"experimental": {
    "modelContextProtocolServers": [
      {
        "transport": {
          "type": "stdio",
          "command": "uv",
          "args": [
            "run",
            "--project",
            "<absolute path project folder>/mcp-server-continue",
            "mcps"
          ]
        }
      }
    ]
  }
}

slash command /echo with @some.file , or any other resource
[ For new or modified features, provide step-by-step testing instructions to validate the intended behavior of the change, including any relevant tests to run. ]

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