feat(acp): support mcp_servers in acp_providers config

[codefromthecrypt]

Allow acp_providers to specify an mcp_servers array that gets passed to create_session. This enables agents to connect to HTTP MCP servers configured by the client.

Example with goose

When using ACP providers, users can now configure MCP servers in their setup:

require("avante").setup({
  provider = "goose",
  acp_providers = {
    ["goose"] = {
      mcp_servers = {
        { type = "http", name = "kiwi", url = "https://mcp.kiwi.com", headers = {} },
      },
    },
  },
})

Note: goose support is pending block/goose#6230