docs(storage): clarify BatchDependentStore invariant and Get semantics

extension/storage/batch_dependent_store.go

@@ -23,13 +23,22 @@ import (
 )
 
 // BatchDependentStore is an interface that defines methods for managing batch dependent information in the database.
+//
+// A BatchDependent is a reverse index ("batches that depend on me") paired one-to-one with a Batch.
+// The batch-creation flow always calls Create here before creating the Batch itself, so every active
+// Batch is guaranteed to have a corresponding BatchDependent row. Lookups via Get are only performed
+// for batch IDs returned from the active-batch set, meaning a missing row indicates data corruption or
+// out-of-band manipulation rather than a normal "not found" outcome. ErrNotFound is therefore part of
+// the contract for completeness but is not expected to be returned in steady-state operation.
 type BatchDependentStore interface {
 	// Get retrieves the batch dependent by batch ID.
-	// Returns ErrNotFound if the batch dependent is not found.
+	// If the batch contains no dependents, the returned BatchDependent will have an empty Dependents list.
+	// Returns ErrNotFound if the batch itself is not found, which should never happen in steady-state system and
+	// therefore does not need a special handling.
 	Get(ctx context.Context, batchID string) (entity.BatchDependent, error)
 
 	// Create creates a new batch dependent.
-	// Returns ErrAlreadyExists if the entry already exists.
+	// Returns ErrAlreadyExists if the entry already exists for the given batch ID.
 	Create(ctx context.Context, batchDependent entity.BatchDependent) error
 
 	// UpdateDependents updates the dependents of a batch dependent and the version to newVersion