validate against meta-schema

[jba]

As part of checking a schema (Schema.checkLocal), it should be validated against the meta-schema.

This involves embedding the meta-schema files for the draft versions we support into the binary, resolving them at startup, and then calling validate on each call to Schema.Resolve.

This isn't essential for a useful package, but it should be done.

[jba]

Comments, from modelcontextprotocol/go-sdk#78:

author:	slimslenderslacks
association:	member
edited:	true
status:	none
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@jba this is upstream of errors like the following, correct?


panic: adding tool "browser_close": cannot validate version http://json-schema.org/draft-07/schema#, only https://json-schema.org/draft/2020-12/schema

We want to support this schema version but it's just not available _today_. Is that right?

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author:	jba
association:	member
edited:	false
status:	none
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We don't have plans to support that version of the spec soon, the jsonschema package will move elsewhere and be maintained independently. At that point, the internal or external contributors would be free to add support for other versions, which this package would then automatically pick up.

This issue is actually about validating schemas against their version, but for now that version has to be draft2020-12.
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author:	slimslenderslacks
association:	member
edited:	false
status:	none
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There are some MCP servers (like playwright and mongodb for example) that fail to load because they need to validate against http://json-schema.org/draft-07/schema#. 

I've been able to get them to work with some updates to the jsonschema package. I can raise a PR to discuss.

[AlekSi]

It looks like there is no API to validate the schema itself against the metaschema. For example, if my schema contains a typo, it will be skipped during the unmarshalling of the schema into the Schema object. I could unmarshal it into the map[string]any instead, but then there is no API to resolve it – I need a Schema object for that. All that is quite unexpected.