Switch `drive +export --file-extension markdown` from the legacy V1
GET /open-apis/docs/v1/content API to the V2
POST /open-apis/docs_ai/v1/documents/{token}/fetch API for
higher-quality Lark-flavored Markdown output.
- Update DryRun and Execute paths to use V2 endpoint with JSON body
- Add docx:document:readonly scope for the new API
- Validate V2 response structure (fail fast on missing document/content)
- Encode token in URL path via validate.EncodePathSegment
- Update unit tests and add V2 response validation error path tests
- Add E2E dry-run test for markdown export path
- Update skill documentation
Allow drive +export to request bitable snapshots with --file-extension base and write them with a .base suffix.
Allow drive +import to accept .base files for bitable only, enforce the 20 MB size limit, and document the new examples and constraints.
Add unit tests for validation and size-limit coverage.
Change-Id: Ia13f5013913812df5fc600c43f90918de4ca6b39
