test: add regression test for dir attribute with siblings (#15126)

[apoorvdarshan]

Summary

• Adds a regression test for Using dir attribute with surrounding content triggers "Cannot read properties of null (reading 'type')" #15126 where elements with dir="auto" caused a compiler crash (Cannot read properties of null (reading 'type')) when sibling content was present
• The bug was fixed as a side effect of compiler refactoring between v5.19.3 and v5.51.3, but no regression test existed

Test plan

• New test passes in all 4 modes: dom, hydrate, ssr, async-ssr
• Covers the exact reproduction from the issue (multiple dir="auto" elements with siblings)

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Latest commit: 160d27d

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[Rich-Harris]

thank you but a test was already added in #15259