Move basic-host and infra, to single sandboxed iframe architecture

[domfarolino]

This PR is a follow-up to #717, implementing a single sandboxed iframe architecture via basic-host, demonstrating a simpler version of what is now compliant per that PR.

Motivation and Context

Updating basic-host in this way is not strictly necessary, but it demonstrates the simplest framing approach as first-class example in this repository, demoting the more complicated double sandboxed iframe approach and reserved sandbox messages.

How Has This Been Tested?

All PDF server tests fail locally on main for some reason. All screenshot tests failed too, for reasons mentioned in #717. Locally, I re-baselined all of the screenshot expectations and they all passed on main, and I used those same re-baselined screenshot expectations when testing this PR, so everything should be good.

Breaking Changes

No.

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
  • Since this PR only modifies examples/, it seems documentation-only.

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

This PR contributes to #716.

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commit: 34b2f13