[combobox] Fix combobox attributes during SSR

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

Ensures the attributes are present before hydration. <Combobox.Trigger> has an existing mismatch if it's not inside the popup, but the issue is minor and can be worked around externally

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/@base-ui/react@4179
  

  pnpm add https://pkg.pr.new/@base-ui/utils@4179
  

commit: 23a3e71

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+16B(0.00%)	🔺+8B(+0.01%)

Details of bundle changes

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[atomiks]

Codex Review

Overview

This PR ensures Combobox ARIA attributes are present during SSR and initial hydration when the input ref is not yet available. It also adds targeted server-rendering tests to lock in expected combobox attributes for input-based and trigger-based configurations.

Findings (None)

No blocking issues found in this patch.

Confidence: 4/5

The behavioral change is small, directly addresses the SSR ref-timing gap, and is backed by focused regression tests on the relevant attribute paths.

Notes

• packages/react/src/combobox/root/AriaCombobox.tsx:969 introduces shouldTreatAsInput so SSR applies combobox ARIA attributes before the input element ref is populated.
• packages/react/src/combobox/root/ComboboxRoot.test.tsx:96 adds SSR regression coverage for default input mode and trigger-based mode.