fix(observable-aliases): backfill missing aliases in v6 data

[PakitoSec]

Summary

• make observable aliases optional in the generated TypeScript types and JSON schema
• default Observable.aliases to an empty list in the Python model
• backfill missing observable aliases when loading existing 6.0.0 investigations and when migrating v5 data
• bump package versions to 6.1.2 and refresh the lockfile

Testing

• add coverage for the generated schema no longer requiring aliases
• add migration tests to verify missing observable aliases are backfilled without mutating the original payload