fix: preserve SPA lazy route errors

[copacabanaservice01]

Summary

• preserve the DataStrategyResult returned by match.resolve() in the non-SSR single-fetch strategy
• allow lazy route module import failures to reach the route error boundary instead of becoming a missing-result error
• add regression coverage and a patch change note

Test plan

• pnpm test packages/react-router/__tests__/router/data-strategy-test.ts --runInBand (27 passed)
• pnpm run --filter react-router typecheck
• pnpm run --filter react-router build
• Prettier and ESLint on the touched TypeScript files
• pnpm run changes:validate

Notes

The broader packages/react-router/ suite passed 113/114 suites locally. The remaining special-characters-test.tsx failures are reproducible on an unmodified origin/main worktree under Windows (10 baseline failures versus 8 on this branch).

Closes #15459