Read and write operations for group_push_rules table

What does this MR do and why?

Implements read and write operations for the new group_push_rules table as part of migrating away from the existing push_rules table for groups.

This is a foundational change for the group push rules migration. When the feature flag is enabled, groups will use the dedicated group_push_rules table instead of sharing the push_rules table with projects.

Key Changes

New Model & Finder

• Adds GroupPushRule model with proper associations to groups
• Creates GroupPushRuleFinder to abstract push rule retrieval logic

Feature Flag Integration

• Introduces the read_and_write_group_push_rules feature flag (disabled by default)
• When enabled, uses new group_push_rules table; when disabled, falls back to existing push_rules table

Controller Updates

• Updates group repository settings and push rules controllers to use the new finder
• Maintains backward compatibility through feature flag checks

Service Layer Changes

• Modifies PushRules::CreateOrUpdateService to handle both old and new models
• Updates predefined rule creation service for groups

API Compatibility

• Updates Group Push Rule API to work with both table structures
• Maintains the same API regardless of which table is used

Helper Method Updates

• Adds safety checks for global? method calls (new model doesn't have this method)
• Updates organization detection logic for push rule inheritance

References

• Epic: Complete `group_push_rules` migration (&15535)

Database

New query in ee/app/finders/group_push_rule_finder.rb

With read_and_write_group_push_rules enabled

🔗 Query plan

SELECT "group_push_rules".*
FROM "group_push_rules"
WHERE "group_push_rules"."group_id" = <group_id>
LIMIT 1

With read_and_write_group_push_rules disabled

It does not introduce new queries.

MR acceptance checklist

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Related to #498936 (closed) and #500481 (closed)