fix(react-router): prevent unstable_sentryVitePluginOptions spread from overwriting sourcemaps

[okxint]

makeCustomSentryVitePlugins builds a sourcemaps: { disable: true, ...user } object to prevent double debug-ID injection during the React Router build. However, the trailing ...unstable_sentryVitePluginOptions spread on the next line overwrites the entire sourcemaps key with the user's raw value, dropping disable: true.

JavaScript's object spread is shallow — when both sourcemaps: { disable: true, ...userSourcemaps } and ...unstable_sentryVitePluginOptions are present on the same level, the second sourcemaps key wins unconditionally.

Fix: destructure sourcemaps out of unstable_sentryVitePluginOptions before the call, then use userSourcemaps inside the already-merged object and spread the remainder separately. This keeps user customizations on all other keys while preserving disable: true.

Fixes #22929