Implement destroy service for v2 approval rules

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Implement destroy service for v2 approval rules.

Requirements:

• Check authorization
• Find the approval rule by ID
• Validate rule can be deleted (only MR-level rules with origin: :merge_request)
• Destroy the rule (cascade deletes join table records and approver associations)
• Return ServiceResponse with success/error

Reference: POC implementation in poc/v2-approval-rules-complete branch