Hide "New group with tab" on tabs that are already in a group

[2nd1st]

Description

The vertical-tab context menu offered "New group with tab" unconditionally. On a tab that is already in a group, selecting it spun the tab into a confusing one-member "New Group" that read as overwriting the current tab. This gates the menu item on self.group_id.is_none(), mirroring the sibling "Remove from group" item which is already gated on the tab being in a group. Regrouping a grouped tab stays reachable via drag / multi-select.

(The grouped-tabs UI is behind the GroupedTabs feature flag.)

Linked Issue

Fixes #13073

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Screenshots are included below.

Testing

• I have manually tested my changes locally with ./script/run

Added a regression test — app/src/tab_tests.rs (tab::tests::new_group_with_tab_hidden_when_tab_already_grouped): with GroupedTabs enabled it asserts tab_group_menu_items omits "New group with tab" for a grouped tab and still offers it for an ungrouped tab. The grouped-tab assertion fails against the old unconditional code.

Screenshots / Videos

Right-click context menu with GroupedTabs enabled:

Tab already in a group — the menu no longer offers "New group with tab" (only "Remove from group", etc.):

Ungrouped tab (for contrast) — still offers "New group with tab":

(Before this fix, the grouped-tab menu also offered "New group with tab" — see #13073.)

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

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Every PR must be linked to a same-repo issue before Oz can review it.

This PR is linked to #13073, but no linked issue is marked ready-to-implement yet. Only repository maintainers apply that label, so please wait for a maintainer to mark the issue. Once it is marked, push a new commit or comment /oz-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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