feat: support Snowflake UNION BY NAME (6994)

[benfdking]

Stacked on sqruff #4241. Merge #4241 first.
#4241

Summary

• Support optional BY NAME after Snowflake UNION, UNION ALL, and UNION DISTINCT.
• Port the upstream UNION BY NAME and UNION ALL BY NAME regression queries and matching parse trees.

Ported from SQLFluff 59bf84573e526900b6d891c0bc05762e7e0ab3be
sqlfluff/sqlfluff#6994
sqlfluff/sqlfluff@59bf845

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test
• bazel test //... --test_timeout=600,300,900,3600 --local_test_jobs=1 --lockfile_mode=off

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-15T10:37:28.200688Z	71f3eb7	PR opened

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" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[github-actions]

Benchmark for 24daaae

Click to view benchmark

Test	Base	PR	%
DepthMap::from_parent	114.1±1.89µs	117.2±1.64µs	+2.72%
fix_complex_query	27.2±0.61ms	24.3±0.38ms	-10.66%
fix_superlong	79.4±4.07ms	74.9±2.32ms	-5.67%
parse_complex_query	4.1±0.07µs	4.1±0.03µs	0.00%
parse_expression_recursion	6.9±0.07µs	6.8±0.17µs	-1.45%
parse_simple_query	1023.7±23.40ns	1038.6±16.42ns	+1.46%