Fix automatic summary container balancing

[puneetdixit200]

Summary

• keep automatic summaries from splitting blockquote, list, and HTML container tags when the word cutoff lands inside them
• extend the automatic summary cutoff to the matching closing tag before ContentWithoutSummary starts
• add unit and integration coverage for blockquotes, lists, and HTML containers

Fixes #14044

Tests

• GOMODCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gomodcache GOCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gocache go test ./resources/page -run TestExtractSummaryFromHTML -count=1
• GOMODCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gomodcache GOCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gocache go test ./hugolib -run TestSummaryAutoBalancesContainerTags -count=1
• GOMODCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gomodcache GOCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gocache go test ./resources/page ./hugolib -count=1
• GOMODCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gomodcache GOCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gocache GOBIN=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gobin PATH=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gobin:$PATH ./check.sh ./resources/page
• GOMODCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gomodcache GOCACHE=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gocache GOBIN=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gobin PATH=/Users/deepakkudi23/ai-resume-contrib-100/hugo-cache/gobin:$PATH ./check.sh ./hugolib
• git diff --check

AI Assistance

I used AI assistance to help prepare this narrow bug fix. I reviewed the implementation and tests before submitting.

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