[SPARK-58975][CORE] Add failure messages to MemoryPool require checks

[uros-b]

What changes were proposed in this pull request?

MemoryPool.incrementPoolSize and decrementPoolSize guard their pool-size accounting invariants with four require(...) checks that carry no failure message. When one fails, the thrown IllegalArgumentException reads only requirement failed, with no indication of which invariant was violated or the values involved.

This adds descriptive messages to those four checks, interpolating the offending values, for example:

require(delta >= 0, s"cannot increment the pool size by a negative amount: $delta")

The sibling MemoryManager already attaches messages to all of its require calls; this brings MemoryPool in line.

Why are the changes needed?

These checks guard internal memory-bookkeeping invariants. A bare requirement failed is hard to act on when one trips; including the invariant and the actual delta / pool size / used-memory values makes such failures diagnosable.

Does this PR introduce any user-facing change?

No. MemoryPool is private[memory]; this only changes the message of an IllegalArgumentException thrown on an internal invariant violation.

How was this patch tested?

Existing tests. The change only adds messages to existing require checks; the conditions themselves are unchanged.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[uros-b]

@srowen Please review.