fix: preserve drag text set by consumers - #1071

Summary

• initialize the Firefox text/plain drag fallback before invoking the consumer callback
• allow onDragStart consumers to set or override the drag text payload
• add a regression test for the call order and final DataTransfer value

Fixes #931.

Why

The Firefox fallback is still required, but applying it after the consumer callback overwrote any text/plain value set by the consumer. Initializing it first preserves the fallback while allowing the public callback to provide the actual payload.

Verification

• exact-base regression failed before the fix: the custom value was followed by an empty fallback
• TreeDraggable.spec.tsx: 37/37 tests passed
• full suite: 14 suites, 225 tests, and 42 snapshots passed
• TypeScript check passed
• lint passed with 0 errors (11 existing warnings)
• compile passed for ESM, CJS, declarations, and CSS
• Prettier check passed for changed files
• git diff --check passed

AI assistance disclosure: Codex was used to trace the drag-start flow, implement the focused fix, add the regression test, and run verification. The behavior and test results above were verified in this branch.

Summary by CodeRabbit

• Bug 修复
  • 修复节点拖拽时自定义拖拽数据可能被默认逻辑覆盖的问题。
  • 现在可正确保留并读取自定义的 text/plain 拖拽内容。