Add support for on-device packages in MCP Registry

[asklar]

• Introduced on_device registry type for pre-installed servers.
• Added optional manifest and __dirname fields in package objects.
• Updated validation logic to accept on-device packages without remote checks.
• Enhanced documentation and examples for on-device server usage.

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

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

Checklist

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

Additional context

[asklar]

thanks @domdomegg! Starting with the manifest (+ __dirname) in the server's _meta sounds good, we can re-evaluate promoting that based on adoption.

Ideally, package would have its own _meta so that we could add it there instead of at the server level. Any idea why that's not present?

I'll update the PR to put it in server._meta for now.

[rdimitrov]

Ideally, package would have its own _meta so that we could add it there instead of at the server level. Any idea why that's not present?

I'll update the PR to put it in server._meta for now.

I think for now we wanted to keep it on the higher level so we don't bloat the format. For example on our side, we handle package specific meta by having a map of extensions under the _meta vendor which we map like - map[package]:{}

[domdomegg]

Hmm some of the MCPB tests are failing in CI - are you able to look into these and fix them?