feat: prevent multiple connections to the same system

[larskemper]

resolves https://github.com/shopware/shopware/issues/13670

This pull request introduces a fingerprinting mechanism for migration connections to prevent duplicate source system connections.

Changes:

• Added a new sourceSystemFingerprint column to the swag_migration_connection table.
• Introduced ConnectionFingerprintService to generate fingerprints for connection credentials, normalize endpoint data, and check for duplicate connections.
• Updated RunService::updateConnectionCredentials to generate and store the fingerprint, and throw a MigrationException if a duplicate source system connection is detected.
• Added a new DUPLICATE_SOURCE_CONNECTION error code and exception to handle duplicate source system connections.