fix(docparser): extract HTML image refs from MinerU Cloud ZIPs

[LHYintheCode]

Summary

• Extract quoted HTML image references from MinerU Cloud ZIP output.
• Add regression coverage for HTML-only MinerU Markdown output.

Why

MinerU Cloud can emit images only as HTML tags, while the ZIP reader previously
collected Markdown image references only. This left valid images unavailable in
document chunks.

Closes #2145

Validation

• go test ./internal/infrastructure/docparser/... -count=1 — exit 0
• go vet ./internal/infrastructure/docparser — exit 0
• git diff --check — exit 0

Review notes

• Draft because the Issue reporter approved the direction, but no maintainer has
  formally assigned or approved it yet.
• Scope is limited to local HTML img src references in MinerU Cloud ZIP output.