Replacing validate uniqueness with db constraint on Sidecar model

[serdarozerr]

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

Changes

• Migration: Cleans duplicate rows, adds sidecars_app_guid_name_index unique constraint (idempotent, with guard check before adding)
• Model: Removed validates_unique [:app_guid, :name], added around_save for DB-level enforcement with a user-friendly error message
• Specs: Migration spec (constraint creation, duplicate cleanup, idempotency, rollback) + updated model uniqueness tests using around_save

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• 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