[SandObserver]

Summary

• Resolve a prerelease tag's notes to its base version's changelog section, falling back to [Unreleased], instead of requiring a section named after the tag.
• Return no notes when a prerelease has nothing pending, so the failure stays explicit.

Testing
All green. Added 1 test covering both fallbacks and the empty case.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.