fix(tabs): Prevent tabs from collapsing into the overflow menu when they exactly fit

[aashutoshrathi]

Since #123879 turned on automatic overflow for every tab list, the event navigation on the issue details page shows only the "First" tab, with the "..." trigger drawn on top of it, at every viewport width. "Latest" and "Recommended" sit in the overflow menu. useOverflowTabs compares the wrapper's clientWidth, which browsers round to a whole pixel, against the sum of the tabs' getBoundingClientRect() widths, which is fractional. That tab list lives in a shrink-to-fit flex item, so the wrapper is exactly as wide as its tabs (for example 223.078px, reported as 223). The check fails by a fraction of a pixel, the last tab is hidden, the wrapper shrinks around the remaining tabs, the ResizeObserver fires and the check fails again, until only the always-kept first tab is left and the budget is negative.

This measures the wrapper with getBoundingClientRect().width so both sides of the comparison come from the same source. The overflow tests now feed the container width through the same mock instead of a clientWidth override, and a new test covers a container that is filled to a fraction of a pixel; it fails without the fix. Verified on the issue details page against the dev UI server and in headless Chromium from 520px to 1440px.

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After (Smaller Screens) [Recommended shortens]

Fixes #124247

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[aashutoshrathi]

@priscilawebdev this is a small follow-up to #123879. After disableOverflow was removed, the issue details event navigation collapses to only "First" with the overflow trigger drawn over it (#124247). The cause is the integer clientWidth versus the fractional tab widths in a shrink-to-fit container; details and a repro are in the issue. Would you mind taking a look when you have a moment? The getsentry dispatch check also needs the Trigger: getsentry tests label from a collaborator.