fix(Menu): correct roles on GroupHeading and Separator

[LukianovII]

Fixes #2090.

Menu.GroupHeading and Menu.Separator both render role="group", so a menu that uses them ends up with empty groups in the accessibility tree — a group whose only content is a heading, and another whose only content is a divider.

https://github.com/huntabyte/bits-ui/blob/main/packages/bits-ui/src/lib/bits/menu/menu.svelte.ts#L1630
https://github.com/huntabyte/bits-ui/blob/main/packages/bits-ui/src/lib/bits/menu/menu.svelte.ts#L1658

Both look like copy-paste from MenuGroupState right above them.

The fix follows conventions already in this repo

• GroupHeading → no role. SelectGroupHeadingState and CommandGroupHeadingState set only an id, and that is enough: the heading names its group through the group's aria-labelledby, which references it by id and does not depend on a role.
• Separator → role="separator". The top-level SeparatorRootState already resolves decorative ? "none" : "separator", so the menu one was the odd one out.

MenuGroupState, MenuRadioGroupState and MenuCheckboxGroupState keep role="group" — correct for those.

Testing

Added a case to dropdown-menu.browser.test.ts asserting the group is the only group, the separator is a separator, the heading carries no role, and the heading still names its group via aria-labelledby. Reverting either line of the fix turns it red.

pnpm -F tests test:browser --browser chromium:

• dropdown-menu — 45 passed
• context-menu + menubar (same menu.svelte.ts) — 49 passed, 1 skipped

pnpm lint clean, prettier --check clean, changeset included (patch).

Note for consumers

Anyone who worked around this by overriding role after spreading props (that was us) can drop the override once this lands — GroupHeading will render no role at all, so a stale role={undefined} override stays harmless either way.

[changeset-bot]

🦋 Changeset detected

Latest commit: e2a1f14

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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[LukianovII]

Test - Chromium is red, but none of the three failures come from this change. Recording the check I did, in case it saves someone the same dig:

failing test	evidence it is pre-existing
combobox > should close on outside click	same close on outside click failure in #2076, #2084, #2073 (select), #2089, #2088 (combobox + popover) — none of them touch menu.svelte.ts
dropdown-menu > should not scroll to previous dropdown trigger when closing a different dropdown	fails identically on #2085, which only renames a Command attribute
context-menu > should allow overriding the pointer events style	ran locally on this branch: context-menu 42/42 passed

All three exhaust retry x3 and emit a -4.png screenshot, i.e. they are timing/scroll sensitive rather than assertion failures.

The case this PR adds (dropdown-menu, roles on group/heading/separator) passes on CI — the only dropdown-menu failure in the run is the scroll one above. Happy to rebase if you'd rather see a clean run after the flakes are addressed separately.