fix(docs): initialize body for partial document data

[hexf00]

Related downstream report: Univer CLI document creation followed by Facade text insertion failed because the created document had no body.

This PR makes partial document data without body initialize as a standard empty document in DocumentDataModel. This is necessary because Facade callers can create documents with only id and title, then naturally call appendText or insertText; before this change the document had no body and those APIs failed with The document body is empty.

The fix keeps a single document creation path by normalizing missing-body document snapshots at the core data-model boundary. It does not add CLI string matching or Facade fallback behavior. Documents that already provide a body keep their supplied body.

How to test them:

• pnpm --filter @univerjs/core exec vitest run src/docs/data-model/__tests__/document-data-model.spec.ts src/docs/data-model/__tests__/empty-snapshot.spec.ts
• pnpm --filter @univerjs/docs exec vitest run src/facade/__tests__/f-document-node.spec.ts
• pnpm --filter @univerjs/core typecheck
• pnpm --filter @univerjs/docs typecheck
• git diff --check

Pull Request Checklist

• Related downstream report is described in the PR description.
• Naming convention is followed.
• Unit tests have been added for the changes.
• No breaking changes introduced in this PR.

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Playwright test results

 28 passed

Details

 28 tests across 12 suites
 7 minutes, 30 seconds
 a9d9f2b
 For more information, see full report and artifacts

[jikkai]

What is the original problem, and is there a way to reproduce it?

[hexf00]

Original downstream issue: https://github.com/dream-num/univer-cli/issues/461

The problem was observed through Univer CLI SaC document authoring. Users could discover univerAPI.createDocument, but there was no safe path for constructing a legal document body. The natural Facade usage is:

const fDocument = univerAPI.createDocument({
  id: 'doc-demo',
  title: 'Demo Doc',
  documentStyle: {},
});

fDocument.appendText('Univer');
// The downstream report used insertParagraph after createDocument in the same missing-body path.

Before this PR, that partial document snapshot kept body missing, so the first text insertion failed with The document body is empty.

The downstream report also contains a second symptom: after users tried to work around the error by hand-writing a minimal body, the handcrafted body had invalid/incomplete document structure and two paragraph insertions collapsed into one paragraph. That is why the downstream issue looks like several separate gaps. The root blocker for this PR is the missing-body partial snapshot: callers should not need to handcraft dataStream / paragraphs / sectionBreaks just to create an editable empty document.

Expected behavior after this PR: DocumentDataModel normalizes document data without body to the standard empty document body (dataStream: '\r\n', one paragraph at 0, one section break at 1), and Facade text insertion works from there. Caller-supplied malformed bodies are not the target of this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.07%. Comparing base (7e8a1ae) to head (a9d9f2b).
⚠️ Report is 3 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7074      +/-   ##
==========================================
- Coverage   62.16%   62.07%   -0.10%     
==========================================
  Files        2592     2595       +3     
  Lines      153097   153725     +628     
  Branches    34999    35178     +179     
==========================================
+ Hits        95177    95428     +251     
- Misses      57920    58297     +377     

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