feat(sidebar): add pinning and close to the right to chat tabs - #14446

What Problem This Solves

The Agent Manager tab bar has Pin and Close to the Right, but the sidebar chat tab strip still only offers Close and Close Others. With several sidebar sessions open there is no way to keep a reference tab in place or clear the trailing tabs in one action.

Why This Change Was Made

Port both actions to the sidebar by reusing the existing Agent Manager logic instead of duplicating the tab components:

• Move the pure pin helpers (applyPinnedTabs, togglePinnedTab) and a new group-aware reorder helper (reorderPinnedTabs) into the shared webview-ui/src/utils/tab-order.ts, and re-export them from the Agent Manager module so existing callers and tests are unchanged.
• Add webview-ui/src/utils/session-close.ts, a small adapter that maps the shared Agent Manager close-to-right helpers onto a session-only tab bar (the sidebar has no terminal or review tabs).
• Add pins and rendered order to LocalTabsProvider, persisted next to the existing tab order as sidebarPinnedSessionTabIDs.
• Wire SessionTabStrip to the existing shared SessionTab and SessionTabMenu, so the sidebar reuses the same components and the existing agentManager.tab.pin / unpin / pinned / closeToRight translations. No new strings were added.

This follows #14444, which added Close to the Right to the Agent Manager tab bar. That PR has merged into main, so this branch is based on main and reuses its helpers directly.

User Impact

• Pinned sidebar tabs render at the front with a pin marker and no close button.
• Close Others and Close to the Right keep pinned tabs open.
• Close to the Right closes only the tabs after the target, hides when nothing closable sits to the right, and keeps the current selection unless the visible tab was closed.
• Pins persist across webview reloads, the same way tab order already does.

Evidence

Automated, from packages/kilo-vscode/: bun run lint, bun run typecheck, bun run knip, and bun run test:unit (6363 pass, 2 skip, 0 fail). New focused coverage for the sidebar lives in tests/unit/session-close.test.ts, plus pin cases in tests/unit/tab-order.test.ts, tests/unit/local-tabs.test.ts, and tests/unit/sidebar-tab-dnd.test.ts.

Manual self-test in an isolated VS Code instance (dev build, real sidebar UI, real sessions created through the backend API so no model requests were sent):

• Pinned a sidebar tab: it moved to the front and showed the pin marker, with a Pinned aria label and no close button.
• Right-clicked a tab: the menu showed Fork Session, Pin, Close, Close Others, Close to the Right in that order.
• Close to the Right on a pinned tab: only the trailing unpinned tab closed, the pinned neighbor survived, and the selection stayed on the visible tab.
• Close to the Right from a tab that was left of the visible tab: the visible tab closed and selection moved to the target.
• The last tab and a tab with only pinned tabs to its right both hid Close to the Right.
• Unpinned a tab and confirmed it dropped out of the pinned group, and pins survived a webview reload.

Pinned tabs at the front, with the trailing unpinned tab still open:

Context menu on a sidebar tab, Close to the Right after Close Others:

After Close to the Right: the trailing tab is gone and both pinned tabs remain:

Close to the Right is absent on the last tab: