fix(react-charts): handle stale legend overflow counts

[Jake Salvesen (jakesalvesen)]

Previous Behavior

Legends could crash while reconciling a dynamic item-count change. When an overflowed list shrank, useOverflowMenu() could briefly return the previous overflowCount; if that count exceeded the current itemIds.length, remainingItemsCount became negative and OverflowMenu dereferenced items[i].props for a missing item.

This produced TypeError: Cannot read properties of undefined (reading 'props') and could unmount the containing chart through its React error boundary.

New Behavior

• Suppresses the overflow menu while its snapshot reports more overflowed items than currently exist.
• Defensively skips a missing overflow item if the arrays are temporarily out of sync.
• Adds a regression test that establishes 14 overflowed legends, shrinks the list from 17 items to 3, and verifies the stale menu is removed while all remaining options stay visible.
• Adds the required patch change file for @fluentui/react-charts.

Validation

• Focused Legends.test.tsx: 13 passed, 4 skipped, 5 snapshots passed.
• react-charts:lint: 0 errors (438 existing package warnings).
• Full react-charts:test: 907 passed, 86 skipped; six unrelated HeatMapChart date snapshots fail locally on Windows because expected Mar/01 / Mar/02 render as Feb/29 / Mar/01.
• Package type-check was attempted, but the local Windows clone could not resolve built workspace package declarations after dependency builds failed on a path-rewrite issue. Upstream CI remains authoritative for that gate.

Related Issue(s)

• Fixes [Bug]: Legends OverflowMenu crashes after the legend count changes (items[i] is undefined) #36641
• Related overflow coverage: Add test for legends overflow scenario #32121 / test(charts): real Legends overflow coverage #36490
• Related fallback-color instability: Implement color fallback to DataVizPalette #31364

[Jake Salvesen (jakesalvesen)]

@microsoft-github-policy-service agree

[Jake Salvesen (jakesalvesen)]

@microsoft/charting-team this is ready for review. The focused Legends regression and package lint pass locally; the PR includes the required @fluentui/react-charts patch change file.