CelestraCloud: bump 2025 → 2026 copyright in CelestraErrorTests files

[leogdion]

Follow-up to PR #298 / branch plans/pr298-fixes. Captured from .claude/ci-failures-pr298.md review-comment "Minor / Nits" section.

Two test files still carry `Copyright © 2025` while the rest of the CelestraCloud sources are on `© 2026`:

• `Examples/CelestraCloud/Tests/CelestraCloudTests/Errors/CelestraErrorTests+Description.swift:6`
• `Examples/CelestraCloud/Tests/CelestraCloudTests/Errors/CelestraErrorTests+RecoverySuggestion.swift:6`

Fix

Edit both files: `© 2025` → `© 2026`.

`./Scripts/lint.sh` (which runs `Scripts/header.sh`) should keep passing afterward; this just brings the headers in line with the rest of the package.