Fix rand() constant folding purity

[officialasishkumar]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number: #6016

Description:

rand() is registered as always pure, so constant folding may evaluate it once and replace expressions such as rand() < 0.1 with a fixed boolean. That makes random sampling predicates return either all rows or no rows.

How do you solve it?

Mark rand() as non-pure, consistent with rand32() and rand64(), so the optimizer does not fold it into a constant. Add a constant-folding unit test to keep rand() calls intact.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Local target build was blocked before reaching this change by an existing GCC 13 -Werror=interference-size warning in MemoryTracker.h. git diff --check passed.

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

Fixed rand() being incorrectly treated as a pure function during constant folding.