Set up public MCP Apps platform tests

[idosal]

Hosts diverge in implementation. The divergence can stem from underspecification, spec changes, non-compliant choices, or simply bugs. Over time, this becomes a challenge to the universal nature of MCP Apps.

We should create a public online host test, similar to the web platforms test, which would help align hosts and inform app developers.

[liady]

We can have it using / coupled to the existing MCP apps debug server.

Some points to consider:

• Run cadence
• Hosts subset (predefined / open)
• Checking auth-gated features

[chelojimenez]

Left a comment on the related discussion. I agree that a debug server is helpful to capture some divergence, but a lot of host behavior can't be probed from the server side. For example, the CSP the Sandbox Proxy actually emits can diverge from what _meta.ui.csp declared, and host implementation strategies like progressive disclosure/caching can't be read from the server.

Happy to share more of how we go about probing hosts for conformance too!