fix: handle mark joins correctly in optimize_projections

[mohanakatari119-bit]

Root Cause

For LeftMark/RightMark joins:

• The output schema is [left columns, mark column] (for LeftMark) or [right columns, mark column] (for RightMark)
• The mark column is synthetic, added by the join itself
• The previous implementation treated LeftMark/RightMark like regular joins
• When split_join_requirements split indices at left_len, it incorrectly tried to route the mark column requirement to the right child
• This caused a schema mismatch because the mark column doesn't exist in either child

Solution

Modified split_join_requirements to handle LeftMark and RightMark joins specially:

• For LeftMark: Route indices before left_len to left child, discard mark column requirement
• For RightMark: Route indices before left_len to right child, discard mark column requirement

Test Plan

Added test exists_or_exists_with_join that:

1. Creates a query with EXISTS(subquery_a) OR EXISTS(subquery_b) filter
2. Follows it with a LEFT JOIN
3. Runs through full optimizer pipeline
4. Verifies no schema mismatch error occurs

Fixes #20083

[mohanakatari119-bit]

@alamb @findepi @viirya @korowa , can you review this.