test: add unit tests for ScmIntegrationService, ScmTokenEncryptionService, and OidcAttestationService

[MaximumTrainer]

Background

Three services have no test coverage:

• ScmIntegrationService — manages SCM (GitHub/GitLab) integration configs.
• ScmTokenEncryptionService — encrypts/decrypts SCM tokens at rest (added in V31 migration). Critical security code.
• OidcAttestationService — handles OIDC-based attestation verification.

Tasks

• Add ScmIntegrationServiceTest covering CRUD operations and conflict/not-found cases
• Add ScmTokenEncryptionServiceTest covering encrypt→decrypt roundtrip and edge cases (empty string, long token)
• Add OidcAttestationServiceTest covering attestation verification happy path and invalid token cases

Files

• backend/src/main/kotlin/com/factstore/application/ScmIntegrationService.kt
• backend/src/main/kotlin/com/factstore/application/ScmTokenEncryptionService.kt
• backend/src/main/kotlin/com/factstore/application/OidcAttestationService.kt