[JSON] Raise exceptions when "sibling" keywords are unhandled

[hudson-ai]

WIP BECAUSE THIS DEPENDS ON STILL-OPEN PR #1061

When an applicator has sibling keywords, JSON schema mandates that both sets of keywords are respected, e.g.

{
   "$defs": {"foo": {"type": "integer"}},
   "$ref": "#/$defs/foo",
   "minimum": 5
}

In this schema, "minimum" is a "sibling" to the "$ref", and we need to enforce BOTH the schema in the definition AND the "minimum" keyword.

There are some trivial examples of this general problem, and we can provide implementations for them (in this PR, I also include an implementation for combinations of the "type", "const", and "enum" keywords, which addresses the enum part of #1038). But when no implementation is provided, we need to raise an exception to prevent "type 1 errors" (the LLM is happy to generate an instance that might not validate against the full schema).

There are a number of tests in the TestRefs class that have to be xfailed due to this stricter behavior. In time, we can add more implementations that handle siblings, reducing the number of xfails.

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Codecov Report

Attention: Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Project coverage is 64.16%. Comparing base (da80081) to head (8487e8b).

Files with missing lines	Patch %	Lines
guidance/library/_json.py	86.84%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1063      +/-   ##
==========================================
- Coverage   65.40%   64.16%   -1.24%     
==========================================
  Files          63       63              
  Lines        4810     4842      +32     
==========================================
- Hits         3146     3107      -39     
- Misses       1664     1735      +71     

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[riedgar-ms]

Will wait on other merge

[hudson-ai]

@riedgar-ms ready :)