[SPARK-58621][ML] Add descriptive messages to SummarizerBuffer metric accessor require checks

[uros-b]

What changes were proposed in this pull request?

Adds a short descriptive message to the nine bare require(requestedMetrics.contains(X)) guards in SummarizerBuffer, so that reading a metric that was not requested fails with a message naming the metric:

require(requestedMetrics.contains(Mean), "mean was not a requested metric.")

Why are the changes needed?

These guards currently produce a bare requirement failed, which does not indicate which metric was involved. The quoted names match the user-facing metric names accepted by Summarizer.metrics(...) per the allMetrics table, including numNonZeros (the metric string differs in casing from the numNonzeros accessor, so the message quotes the name a caller actually passes).

These are plain require calls rather than part of the structured error-condition framework, and the adjacent require(totalWeightSum > 0, ...) check in each of the same methods already carries a message, so this brings the metric guards in line with their immediate neighbours.

Does this PR introduce any user-facing change?

No. SummarizerBuffer is private[spark], and only the message text of an existing check is added.

How was this patch tested?

Conditions and exception types are unchanged, so existing tests continue to apply; the error-handling tests in SummarizerSuite assert only the exception type and do not match on message text.

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

Generated-by: Claude Code (Opus 4.8)

[uros-b]

Waiting for CI. @zhengruifeng Please review.