Add ui/close-resource request for UI to initiate termination

[fredericbarthelet]

Fixes #203

Motivation and Context

Implement bi-directionality on UI termination.
Until now ui/resource-teardown was the only way for Host to initiate UI teardown.
This methods enable UI to initiate its own teardown

Items that might require discussion in this PR:

• naming. Similarly to open-link, message, request-display-mode, the name of the request should be action-first/verb-first. Even if we implement a fire and forget pattern (see next point), I would not include notifications in the name. What do you think of ui/close-resource?
• should this new ui/close-resource message implements a request/response pattern or a fire and forget type of pattern (where UI fires the event like it would fire a notification without ever expecting an answer). I'm leaning towards fire and forget because the host won't be able to send a response if it proceeds accordingly and does indeed terminates the UI
• should we provide a way for the host to refuse terminating the UI or should all ui/close-resource requests be fulfilled? this can be achieved only if we go for a request/response pattern, so I'd not account for this scenario just yet

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/ext-apps@215

commit: 52fff75

[fredericbarthelet]

Hey @liady @idosal
Would love to get your feedback on this first shot at ui/close-resource before going further and adding tests and examples.
Let me know what you think (I raised 3 main points in the PR description worth discussion IMHO) :)