Change Summary

Adds JSON-Schema handler for ImportString which failed to schematize prior to this change. The schema type of importstring is simply string (which gets imported on validation).

There was a test specifically for an exception for this type, which was changed to validate a model containing an importstring type - there is another test already exercising a bad json schema using the same exception type (see https://github.com/pydantic/pydantic/blob/main/tests/test_types.py#L6155), so there does not look to be loss here.

Note: There could be an opportunity to use a regex validation on the string to match something importable, but I left that out of scope here.

Related issue number

Fix #9327

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable - I think the documentation currently implies json schema should work (by omission that it explicitly should not)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @dmontagu