fix(react-pdf): draw a repeat container once in renderToBuffer

[peter-cynomi]

Problem

The server walk behind renderToBuffer, renderToStream and renderToFile draws a repeat container once per item. @json-render/react, and react-pdf's own provider Renderer, draw the container once and put every item's children inside it.

In the spec below, the outer section is repeated over /domains, and each domain has a nested Tasks section repeated over { "$item": "tasks" }. The server walk draws a different structure from React:

Line	@json-render/react / provider Renderer	renderToBuffer on main
Appendix B (outer repeat container title)	once	once per domain
Tasks (inner repeat container title)	once per domain, including a domain with no tasks	once per task, and none for the empty domain

The effects:

• A repeat container with its own content, such as a section title or a table header, is duplicated in the PDF.
• An empty repeat draws nothing, where React still draws the container.
• React logs a missing key warning, because the per-item containers are returned in an unkeyed fragment.

Fix

The server walk now draws the container once, with each item's children in a keyed React.Fragment. Item keys still use repeat.key, falling back to the index.

Tests

packages/react-pdf/src/repeat-container.test.tsx runs a nested-repeat spec through both walkers and compares the order and count of the drawn lines. It also checks that an empty repeat draws its container once.

Both walkers pass on this branch. On main, the renderToBuffer cases fail and the provider Renderer cases pass. tsc --noEmit is clean and @json-render/react-pdf builds.

Note

This touches the same repeat block in render.tsx as #364, which filters repeat items by $item in visible. Whichever PR merges second will need a small rebase.

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