Represent ternary expressions with a dedicated parsetree node

[cknitt]

Refs #8624

Summary

• Add Pexp_ternary with three required operands, so a ternary cannot lack an else branch.
• Keep the existing res.ternary encoding only in the v0 PPX compatibility bridge.
• Update typing, printing, AST consumers, and syntax snapshots. Preserve parentheses when a ternary is another ternary's condition.
• Add bridge tests for round trips, attribute order, and incomplete legacy if nodes.

[codecov]

Codecov Report

❌ Patch coverage is 77.51938% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.57%. Comparing base (58c6c89) to head (ceb6442).

Files with missing lines	Patch %	Lines
compiler/syntax/src/res_parsetree_viewer.ml	36.36%	7 Missing ⚠️
compiler/ml/typecore.ml	37.50%	5 Missing ⚠️
compiler/ml/printast.ml	0.00%	4 Missing ⚠️
compiler/syntax/src/res_parens.ml	50.00%	4 Missing ⚠️
tests/ounit_tests/ounit_ast_mapper0_tests.ml	90.00%	4 Missing ⚠️
analysis/src/hint.ml	33.33%	2 Missing ⚠️
analysis/src/utils.ml	0.00%	1 Missing ⚠️
compiler/syntax/src/res_comments_table.ml	80.00%	1 Missing ⚠️
compiler/syntax/src/res_printer.ml	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8674   +/-   ##
=======================================
  Coverage   78.57%   78.57%           
=======================================
  Files         476      476           
  Lines       64157    64232   +75     
=======================================
+ Hits        50414    50473   +59     
- Misses      13743    13759   +16     

Files with missing lines	Coverage Δ
analysis/src/xform.ml	88.35% <100.00%> (+0.08%)	⬆️
compiler/frontend/bs_builtin_ppx.ml	91.37% <100.00%> (+0.05%)	⬆️
compiler/ml/ast_helper.ml	80.79% <100.00%> (+0.06%)	⬆️
compiler/ml/ast_iterator.ml	93.89% <100.00%> (+0.05%)	⬆️
compiler/ml/ast_mapper.ml	77.58% <100.00%> (+0.09%)	⬆️
compiler/ml/ast_mapper_from0.ml	72.26% <100.00%> (+0.40%)	⬆️
compiler/ml/ast_mapper_to0.ml	71.48% <100.00%> (+0.28%)	⬆️
compiler/ml/depend.ml	83.38% <100.00%> (+0.15%)	⬆️
compiler/ml/pprintast.ml	91.33% <100.00%> (+0.03%)	⬆️
compiler/syntax/src/res_ast_debugger.ml	96.22% <100.00%> (+0.03%)	⬆️
... and 10 more

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 86f0ba9278

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 8e699aa5f7

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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commit: ceb6442

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8674

[cristianoc]

Nice cleanup. Fewer and fewer dark corners left.