[Backport 18.9] Fix gitlab:setup failure on fresh database

What does this MR do and why?

Contributes to #591292 (closed)

Problem

Running gitlab:setup on a fresh PostgreSQL database fails with PG::UndefinedTable: ERROR: relation "feature_gates" does not exist.

This regression was introduced in 18.9 by !220200 (merged) which added a circuit breaker to the Gitaly client. The CircuitBreaker#enabled? method calls Feature.enabled? before the database schema is initialized, causing Flipper to query the non-existent feature_gates table.

Solution

Check Feature::FlipperFeature.table_exists? before calling Feature.enabled? in CircuitBreaker#enabled?. This follows the established pattern used in lib/feature/gitaly.rb.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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  • severity label and bug subtype labels (if applicable)
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• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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