Support MCP Tasks

**What problem would this solve?**
* goose doesn't yet support the new Tasks primitive in MCP https://modelcontextprotocol.io/extensions/tasks/overview
* They are included in the 2026-07-28 spec as an official extension (like Apps), and seem on a direction to go into the core in a future version of MCP. We should support them.

**What would a good outcome look like?**
* Tasks and their full lifecycle are supported from an MCP client perspective in goose
* Long-running operations can be modeled by this

**Possible approaches**
* Use the existing rmcp support for them in the 3.x copy of the library and add the goose-side handling and light UI for interacting with tasks

**Additional context**
N/A

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).