feat(docs): support --replace --markdown --tab whole-tab re-render (#595)

[sebsnyk]

Summary

Closes #595.

gog docs write <docId> --replace --markdown --tab=<tab> previously errored with --markdown with --replace does not support --tab (Drive's markdown converter operates on entire documents). Drive's converter is whole-document-only, but the Docs API itself happily targets tab ranges, so the whole-tab re-render is a small workaround at the gogcli layer:

1. Look up the tab's end index via documents.get with includeTabsContent=true.
2. If the tab has any body content (endIndex > 2), issue a DeleteContentRange covering [1, endIndex-1] with the tab ID.
3. Render the markdown locally and submit via the existing insertDocsMarkdownAt helper at index 1 with the tab ID — the same path that already powers --append --markdown --tab (including bullets, tables, code, images, and inline formatting).

No new dependencies; reuses ParseMarkdown / MarkdownToDocsRequests / TableInserter / insertImagesIntoDocs, all of which already accept a tab ID. Empty tabs skip the delete step; unknown --tab values still produce the standard tab-not-found error before any batchUpdate.

gog docs write <docId> --replace --markdown --tab "Gold list" --file gold.md
# Replaces only the Gold list tab content with the markdown-rendered version.
# Other tabs are untouched.

Test plan

• go build ./... clean.
• go test ./... clean (43.8s on internal/cmd).
• New unit tests in internal/cmd/docs_write_markdown_tab_test.go:
  • TestDocsWrite_MarkdownReplaceWithTab — asserts a non-empty tab produces DeleteContentRange{Range:{Start:1,End:19,TabId:t.second}} followed by InsertText{Location:{Index:1,TabId:t.second}} + formatting requests all carrying the tab ID; verifies Drive is never called.
  • TestDocsWrite_MarkdownReplaceWithTab_EmptyTab — asserts the delete step is skipped when the tab body is empty (Docs API rejects end <= start ranges).
  • TestDocsWrite_MarkdownReplaceWithTab_TabNotFound — asserts an unknown tab title produces the standard tab not found: "Missing" error before any batchUpdate is issued.
• CHANGELOG.md updated under 0.17.1 Added.
• docs/docs-editing.md updated with the new tab-rerender example.

Notes for reviewer

• Help text for --markdown was not changed; its existing wording (requires --replace or --append) is still accurate.
• Pageless mode continues to apply post-write when requested; it's a document-level property, not per-tab.
• The error string --markdown with --replace does not support --tab (...) was removed since the case it described is now supported.

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