ENG-1405 Fix restore testing plan name: replace hyphens with underscores and remove tautological suffix - #48

Description

Fixes the restore_testing_plan_name in the source module to comply with the AWS Backup API constraint that only allows alphanumeric characters and underscores. Replaces hyphens in local.resource_name_prefix with underscores via replace(), and removes the redundant _backup_restore_testing_plan suffix that consumed 28 of the 50-character limit without adding value.

Context

The AWS RestoreTestingPlanName field must consist of only alphanumeric characters and underscores (max 50 chars). The previous implementation either appended a tautological suffix (_backup_restore_testing_plan) or fell back to a hardcoded name, and did not handle hyphens that naturally appear in resource_name_prefix (e.g. from region names like eu-west-2). This caused failures for environments with longer prefixes (e.g. mesh) where the 50-char limit was exceeded.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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