Handle zero and negative SequenceSet min/max counts

[OskarEichler]

Summary

Return an empty SequenceSet for max(0), reject negative min/max counts, and normalize integer-like counts before slicing. max(0) currently slices from -0 and returns the whole set; negative counts produce unrelated slices. Preserve oversized-count and frozen-result behavior.

Reproduction

require 'net/imap'
set = Net::IMAP::SequenceSet['1,3,5']
p set.max(0).to_s # before: "1,3,5"; after: ""
begin
  set.min(-1)
rescue ArgumentError => e
  p e.message # "negative count"
end

Verification

• 200 focused checks; 34 failing expectations before, zero afterward. Empty/mutable/frozen/star sets, zero/positive/oversized/negative counts and objects implementing to_int are covered. Existing ✨ Add SequenceSet#min(count) and #max(count) #460 defines count behavior and 🐛 Fix SequenceSet#max(n) when cardinality < n <= size #580 fixes a separate cardinality/duplicate case already present in master.
• Existing rake test on this isolated branch: 1726 tests, 12598 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications, Ruby 4.0.6 via rbenv. The baseline also passes all 1,726 tests; assertion counts vary slightly across runs.
• Supplemental RuboCop Lint retains the same 48 existing findings. Ruby syntax and git diff --check pass. No repository tests, dependencies or workflows were added or modified; focused reproductions live outside the repository under the consumer's no-new-tests policy.
• Independent branch based on 6d2ef7a636a1e2449187a83b06ac7a5baa54ead2; the runtime diff from released 0.6.6 is documentation-only before this patch.

Compatibility and limits

Intentional correction: negative counts now raise ArgumentError rather than returning a slice. Zero yields an empty set. Omitted/nil/false count behavior and scalar star handling are unchanged. Integer-like counts use the existing SequenceSet to_int coercion convention. Other Ruby/OS versions were not run locally. No production or external IMAP service was used. Local verification does not imply upstream CI approval or comprehensive behavioral coverage.