Add minimum floor to Elastic migration throttle_delay

What does this MR do and why?

A migration declaring throttle_delay <= 1 second can be blocked by its own self-enqueued successor. The exclusive-lease TTL is [throttle_delay.to_i - 1, 1].max, which equals 1 when throttle_delay is 1 second. The lease can still be held when the successor arrives, inverting the throttle it is meant to apply.

This MR adds MINIMUM_THROTTLE_DELAY = 2.seconds and enforces it in the throttle_delay class method so the lease TTL formula always produces a value strictly less than the delay. No existing migration declares a delay below 5 seconds, so this is a preventive guard.

Changes

• ee/app/workers/concerns/elastic/migration_options.rb: add MINIMUM_THROTTLE_DELAY = 2.seconds constant; raise ArgumentError in throttle_delay(value) when the value is below the minimum.
• ee/spec/workers/concerns/elastic/migration_options_spec.rb: two new examples — one asserting the error is raised for a sub-minimum value, one asserting the minimum itself is accepted.

How to verify

bundle exec rspec ee/spec/workers/concerns/elastic/migration_options_spec.rb

MR acceptance checklist

• This MR has been reviewed against the MR acceptance checklist.
• This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.

Changelog: fixed Closes #630966