[OpenAI] Image inputs and JSON structured outputs

[hudson-ai]

Punchline:

Details:

• improved dispatching of guidance AST nodes to relevant client handlers (we got JSON in OpenAI!)
• fudged workaround for displaying role tags in the vis (@nopdive -- let's iterate on this in the next PR, but things look a lot better!)
• some fixes/improvements based on @mmoskal's suggestions in Replace existing grammar AST with a more explicitly rule-based AST #1128

Probably not quite "release ready", but we're getting close. I may go take a nap.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 57.55102% with 104 lines in your changes missing coverage. Please review.

Project coverage is 47.24%. Comparing base (9fe8b26) to head (5512b0f).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
guidance/models/_openai.py	14.75%	52 Missing ⚠️
guidance/_ast.py	75.71%	17 Missing ⚠️
guidance/models/_base/_client.py	66.66%	12 Missing ⚠️
guidance/models/_engine/_client.py	76.19%	10 Missing ⚠️
guidance/library/_gen.py	60.00%	6 Missing ⚠️
guidance/models/_base/_model.py	50.00%	5 Missing ⚠️
guidance/_grammar.py	88.88%	1 Missing ⚠️
guidance/library/_image.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1134      +/-   ##
==========================================
+ Coverage   43.44%   47.24%   +3.80%     
==========================================
  Files          72       78       +6     
  Lines        5849     5522     -327     
==========================================
+ Hits         2541     2609      +68     
+ Misses       3308     2913     -395     

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[nopdive]

Outside of image inputs and JSON struct outputs it fixes previous visualization bugs that we mentioned will be pushed into main. Approved, merge when ready.