feat(core): add validationOnly mode to prevent change execution

[bercianor]

Summary

• Adds a validationOnly flag that makes Flamingock verify pending changes without executing them — throws PendingChangesException if any are found, continues normally if everything is up to date
• Designed for production safety patterns (e.g. Spring Boot profiles where validation-only: true in prod prevents accidental change execution)

Changes

New files

• PendingChangesException — extends FlamingockException, carries pendingCount
• ValidateOperation — mirrors ExecuteOperation loop but throws instead of applying; acquires lock for consistency guarantees

Modified files

• CoreConfigurable / CoreConfiguration — validationOnly flag (default false)
• OperationFactory — routing override: EXECUTE_APPLY + isValidationOnly() → EXECUTE_VALIDATE; implements the pre-existing orphaned OperationType.EXECUTE_VALIDATE
• AbstractChangeRunnerBuilder — fluent setValidationOnly(boolean)
• SpringbootProperties — delegates flamingock.validation-only to coreConfiguration

Usage

Java:

Flamingock.builder()
    .setValidationOnly(true)
    .build()
    .run();

Spring Boot YAML:

flamingock:
  validation-only: true

Behaviour

• No pending changes → continues normally, result NO_CHANGES
• Pending changes found → throws PendingChangesException: "Flamingock validationOnly=true: N pending change(s) detected"
• Lock is always acquired — validation is consistent (another instance cannot execute changes concurrently while this one validates)

Testing

• PendingChangesExceptionTest — count, message, type hierarchy (5 tests)
• ValidateOperationTest — no pending → success; pending → exception (3 tests)
• OperationFactoryTest — routing to ValidateOperation when flag is active (2 tests)
• Spring Boot end-to-end integration test (2 tests)