Replacing validate uniqueness with db constraint on buildpack model

[serdarozerr]

Replaces the Sequel validates_unique check with a DB-level unique index on (name, stack, lifecycle). The around_save hook catches UniqueConstraintViolation and converts it to ValidationFailed, making uniqueness enforcement race-condition safe.

Changes

• Migration: Cleans duplicate rows, drops old unique_name_and_stack index, adds new buildpacks_name_stack_lifecycle_index (idempotent)
• Model: Removed validates_unique, added around_save for DB-level enforcement
• Specs: Migration spec (index swap, idempotency, duplicate cleanup) + updated model uniqueness tests

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• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[johha]

LGTM