[6.x] Draw a slotted secondary-nav action inside the menu

[brianjhanson]

A secondary nav whose actions come from the actions slot rather than the actions prop now draws a menu, and draws the slotted action inside it.

Two things stood in the way:

SecondaryNav gated the menu on actions.length, so filling only the slot produced no menu at all and the slotted content never rendered. The guard now also counts the slot.

Above the large breakpoint the slot rendered beside ActionMenu rather than inside it, so a slotted action sat loose next to the menu button instead of joining the items under it. ActionMenu gained an actions slot of its own, appended after its rendered list, and the nav passes its slot through to it. The collapsed presentation already did this — its slot sits inside the menu's content — so the two now agree.

Verification

A nav given only prop actions is unchanged, in both presentations. One given only a slotted action now shows the menu, with the action inside it. SecondaryNav's tests pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NjeigCjRgjTJpnhbvyrxv8

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[brianjhanson]

Rolled into #19884 — ActionMenu.vue and SecondaryNav.vue there are byte-identical to this branch, since the bridged element index needs the slotted action to land inside the menu.