Implement gitlab:db:validate_schema rake task with exit-code contract

Summary

Implement (or extend) the gitlab:db:validate_schema rake task with a well-defined exit-code contract so that omnibus-gitlab's preinst can use it as a pre-upgrade DB schema consistency gate.

This is a prerequisite for the omnibus-gitlab warn-only release. The GitLab version that ships this task becomes SCHEMA_CHECK_MIN_SOURCE_VERSION in omnibus.

Exit-code contract

Exit code	Meaning	Phase 1 (warn-only)	Phase 2 (blocking)
0	Schema consistent with db/structure.sql	Continue	Continue
1	Inconsistencies found; details on stdout/stderr	Warn and continue	Block (unless bypassed)
2	Operational error — couldn't connect/boot/run	Warn and continue	Warn and continue (never block)

Exit 2 is reserved for the blocking release but should be emitted from the start so Phase 2 requires no follow-up upstream change.

Task name

gitlab:db:validate_schema (confirmed, no namespace change).

Acceptance criteria

• Exits 0 when the live DB schema is consistent with db/structure.sql
• Exits 1 with human-readable inconsistency detail on stdout/stderr when drift is detected
• Exits 2 for operational errors (DB unreachable, Rails boot failure, etc.)
• RSpec tests: clean schema → exit 0, injected drift → exit 1 with detail output
• The introduced GitLab minor version is noted in the MR description (needed for SCHEMA_CHECK_MIN_SOURCE_VERSION in omnibus)

Scope

• main DB only (Geo geo DB is out of scope)
• No auto-remediation — report only

Part of

Epic: gitlab-org#22667