feat(docreader): add XMind outline parsing

[guiyangyuan]

Description

Add native .xmind support to the built-in DocReader so mind maps can be uploaded and indexed without a manual Markdown or image export.

• Parse modern XMind archives from content.json and classic XMind 8 archives from content.xml using only the Python standard library.
• Preserve sheet and topic hierarchy as Markdown outlines, including plain-text node notes.
• Prefer modern JSON when both formats are present and reject malformed, encrypted, empty, or oversized content entries with clear errors.
• Advertise xmind through both the Python parser registry and Go engine registry so existing clients can discover support.
• Add parser, validation, routing, and engine-registry regression tests.

User impact: teams can directly add structured XMind knowledge to WeKnora for RAG, search, and summarization workflows.

Breaking changes: none.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Closes #2705

Testing

uv run --project docreader --no-sync python -m unittest discover -s docreader/tests -p "test_*.py" -v
uv run --project docreader --no-sync python -m compileall -q docreader
go test ./internal/infrastructure/docparser -count=1

Results: 157 Python tests passed with 12 environment-dependent fixture tests skipped; Python compilation and Go document-parser tests passed.

Checklist

• git diff --check upstream/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable (for Go: golangci-lint run --new-from-rev=origin/main ./...)
• Full DocReader Python tests and scoped Go tests were run
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

Not applicable; this change adds backend document parsing and engine discovery without UI changes.

Out of Scope

Images, labels, markers, priorities, relationships, and graphical rendering are intentionally left for follow-up work.