Use json_string kwarg from llguidance 0.1.7to properly escape json string patterns

[hudson-ai]

• Pass json_string down into Lexeme/llguidance
• Rewrite json_string definition in library/_json.py
• Add some tests to ensure that user specified patterns like .* can only produce valid json strings

[hudson-ai]

@mmoskal a quick glance-over from you would be appreciated :)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 61.28%. Comparing base (958145c) to head (2394cc8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
guidance/_grammar.py	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
- Coverage   70.11%   61.28%   -8.84%     
==========================================
  Files          62       62              
  Lines        4421     4422       +1     
==========================================
- Hits         3100     2710     -390     
- Misses       1321     1712     +391     

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[mmoskal]

looks great!