Support boolean JSON schemas

[hudson-ai]

We were previously supporting True/False schemas only when nested in certain places like items and additionalProperties. This expands our coverage to handle top-level boolean schemas as well.

Note that we will now raise a ValueError for False schemas, simply because there is nothing we can generate in this case. Alternatively, we return a null grammar... But this may have unforseen consequences.

Added some minimal tests, as we already have decent coverage for the nested cases in the TestEmptySchema class.

Note: developed alongside #1014, which made it much easier to tell whether I had full coverage of every type we accept :)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 61.33%. Comparing base (418fc03) to head (2ea9d50).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
guidance/library/_json.py	87.50%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1015      +/-   ##
==========================================
- Coverage   70.06%   61.33%   -8.73%     
==========================================
  Files          62       62              
  Lines        4443     4449       +6     
==========================================
- Hits         3113     2729     -384     
- Misses       1330     1720     +390     

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[hudson-ai]

Will merge if there are no objections @Harsha-Nori & @riedgar-ms

[Harsha-Nori]

thoughts on raising error vs. returning empty string?

[Harsha-Nori]

Intuitively I feel like warning + empty string is more "correct", but I'm easily convinced otherwise.

[hudson-ai]

The empty string isn't valid against the False schema. I think we need to raise an exception if someone wants to generate against a schema with no valid instances. Another "bad" schema by this standard is

{
  "type": "object",
  "properties": {
    "name": false
  },
  "required": ["name"]
  }

Note that raising an exception upon encountering a false subschema isn't actually correct though. See the following example:

{
 "type": "object",
 "properties": {
   "name": false
 },
 "required": []
 }

This should validate against any object that does not have the key "name". Will not support for now -- I think I'll open an issue against this just to track it...

Edit: issue #1018