feat: Add Agentic MCP support with a new chat/completion endpoint

[mudler]

Description

This PR fixes #6380

Notes for Reviewers

To test, add for instance in a model config the following section:

mcp:
  stdio: |
    {
      "mcpServers": {
        "searxng": {
          "command": "docker",
          "args": [
            "run", "-i", "--rm",
            "quay.io/mudler/tests:duckduckgo-localai"
          ],
        }
      }
    }

Afterwards, you should be able to call the mcp endpoint (/mcp/v1/chat/completions - which uses the agent to answer, accessing the tools made available from the MCP servers configured) with:

curl http://localhost:8080/mcp/v1/chat/completions -H "Content-Type: application/json" -d '{
  "model": "qwen3-4b",
  "messages": [{"role": "user", "content": "what are the latest news about Trump?"}]
}'

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