Summary

• Fix EXCEPT ALL/DISTINCT and MINUS ALL/DISTINCT modifiers being silently dropped during parsing
• Reset the modifier variable between set-operation loop iterations to prevent modifier leaking from one operator to the next

Problem

The grammar in JSqlParserCC.jjt captures the set operation modifier via SetOperationModifier() but constructs ExceptOp and MinusOp with their no-arg constructors (which default to empty string), discarding the captured modifier. UnionOp and IntersectOp correctly receive modifier as a constructor argument.

// CORRECT: modifier passed to constructor
<K_UNION> [ modifier=SetOperationModifier() ] { UnionOp union = new UnionOp(modifier); ... }
<K_INTERSECT> [ modifier=SetOperationModifier() ] { IntersectOp intersect = new IntersectOp(modifier); ... }

// BUG: modifier NOT passed to constructor
<K_MINUS> [ modifier=SetOperationModifier() ] { MinusOp minus = new MinusOp(); ... }
<K_EXCEPT> [ modifier=SetOperationModifier() ] { ExceptOp except = new ExceptOp(); ... }

Additionally, the modifier variable was not reset between iterations of the (LOOKAHEAD(2) ...)+ loop, so a UNION ALL ... EXCEPT would incorrectly leak the ALL modifier to the EXCEPT.

Fix

1. Pass modifier to MinusOp(modifier) and ExceptOp(modifier) constructors
2. Add { modifier = null; } at the start of each loop iteration

Test plan

• testExceptAllRoundTrip - verifies EXCEPT ALL round-trips correctly
• testExceptDistinctRoundTrip - verifies EXCEPT DISTINCT round-trips correctly
• testMinusAllRoundTrip - verifies MINUS ALL round-trips correctly
• testMinusDistinctRoundTrip - verifies MINUS DISTINCT round-trips correctly
• testModifierDoesNotLeakFromUnionAllToExcept - verifies modifier doesn't leak across operators
• testModifierDoesNotLeakFromIntersectAllToUnion - verifies modifier doesn't leak across operators
• testMixedModifiersPreserved - verifies mixed modifiers in chained set ops
• testExceptAllSetOperationObject / testMinusAllSetOperationObject - verifies SetOperation state
• Full test suite passes (4671+ tests, 0 failures)

Fixes #2419

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