parser: fold set-op keywords case-insensitively (mixed-case arms were dropped)

[chiradip]

Problem

fold_set_operations decided the set operator by comparing the case-preserved token text against only the UPPER and lower spellings:

if (keyword == "UNION" || keyword == "union") { ... }
else if (keyword == "EXCEPT" || keyword == "except" || keyword == "MINUS" || keyword == "minus") { ... }
// and for INTERSECT: if (!(keyword == "INTERSECT" || keyword == "intersect")) break;

SQL keywords are case-insensitive, so a mixed-case spelling like Union / Intersect / ExCePt matched neither branch. The fold loop then breaks — and the operator plus the entire right operand are silently dropped:

Input	Before	After (correct)
SELECT 1 Union SELECT 2	lone SELECT 1 (right arm gone)	UNION(SELECT 1, SELECT 2)
SELECT 1 UNION SELECT 2 Intersect SELECT 3	UNION(1, 2) — INTERSECT arm gone, wrong shape	UNION(1, INTERSECT(2, 3))
SELECT 1 ExCePt SELECT 2	lone SELECT 1	EXCEPT(SELECT 1, SELECT 2)

This is a silently-wrong result: a legal multi-arm set query is truncated to its first branch with no diagnostic. Uppercase and all-lowercase both worked; only mixed case failed. Found by the repeatable frontend-audit's regression sweep of the INTERSECT-precedence set-op fold.

Fix

Match on the case-insensitive current_token_->keyword_id (the tokenizer already sets Keyword::UNION / INTERSECT / EXCEPT for any casing) instead of the case-preserved spelling. The original spelling is still copied into primary_text, so the node's displayed keyword keeps the user's casing.

MINUS is not a set-op keyword in this tokenizer (there is no Keyword::MINUS), so it is never a Keyword-typed token and never reached this Keyword-gated loop — the old "MINUS" string branch was dead code. Dropping it changes nothing (verified: ... users MINUS SELECT ... still parses exactly as before, with MINUS consumed as a table alias).

Tests

Adds three regression tests to test_precedence_regression.cpp:

• MixedCaseUnionFolds — Union keeps both arms
• MixedCaseExceptFolds — ExCePt folds as EXCEPT
• MixedCaseIntersectBindsTighterThanUnion — mixed-case inner Intersect preserves both the fold and the precedence

Full suite: 37/37 green.