Fix panic when topk() reduces series below maxSeries in QueryRange

[Syedowais312]

Fixes #6000

Summary

Fixes a crash in QueryRange when the internal series limit is reached while using topk() or bottomk().
These functions reduce the series count before truncation, causing an out-of-bounds slice.

Root Cause

MaxSeriesReached() triggers truncation using:

resp.Series = resp.Series[:maxSeries]

But after topk(10), the response may contain fewer series than maxSeries, leading to:

panic: slice bounds out of range

Fix

Added a conditional check:

if len(resp.Series) > maxSeries {
    resp.Series = resp.Series[:maxSeries]
}

This prevents slicing beyond available series.

Tests

• Verified build: go build ./pkg/traceql
• Ran combiner tests: go test ./modules/frontend/combiner
• Tests pass.

[mdisibio]

Thanks for taking care of this. Fix looks good, can you add a changelog.md entry? And we'll merge.

[Syedowais312]

Thanks for taking care of this. Fix looks good, can you add a changelog.md entry? And we'll merge.

Changelog entry added. Let me know if anything else is needed!
Thank you for the review!