autoresearch iter 1: keep Using2/Using3 dependency causes reachable

Using2/Using3 flattened per-dependency injection failures with %v while
Using1 wrapped with %w, so errors.Is could not see ErrServiceNotFound
through a multi-dependency resolution failure.

backend/core/container.go

@@ -5,6 +5,7 @@
 package core
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"sync"
@@ -179,8 +180,8 @@ func Using[T1 any](ctx *Context, fn func(s1 T1)) error {
 func Using2[T1, T2 any](ctx *Context, fn func(s1 T1, s2 T2)) error {
 	s1, err1 := Inject[T1](ctx)
 	s2, err2 := Inject[T2](ctx)
-	if err1 != nil || err2 != nil {
-		return fmt.Errorf("%w: (dep1: %v, dep2: %v)", ErrServiceNotReady, err1, err2)
+	if err := errors.Join(err1, err2); err != nil {
+		return fmt.Errorf("%w: %w", ErrServiceNotReady, err)
 	}
 	fn(s1, s2)
 	return nil
@@ -191,8 +192,8 @@ func Using3[T1, T2, T3 any](ctx *Context, fn func(s1 T1, s2 T2, s3 T3)) error {
 	s1, err1 := Inject[T1](ctx)
 	s2, err2 := Inject[T2](ctx)
 	s3, err3 := Inject[T3](ctx)
-	if err1 != nil || err2 != nil || err3 != nil {
-		return fmt.Errorf("%w: (dep1: %v, dep2: %v, dep3: %v)", ErrServiceNotReady, err1, err2, err3)
+	if err := errors.Join(err1, err2, err3); err != nil {
+		return fmt.Errorf("%w: %w", ErrServiceNotReady, err)
 	}
 	fn(s1, s2, s3)
 	return nil

backend/core/context_test.go

@@ -222,6 +222,33 @@ func TestContextUsingMultiple(t *testing.T) {
 	assert.True(t, called3)
 }
 
+// UsingN must keep every dependency failure reachable through the error chain,
+// not just report that something went wrong.
+func TestContextUsingMultipleErrorChain(t *testing.T) {
+	ctx := core.NewContext(context.Background())
+
+	err := core.Using2(ctx, func(s SampleService, l LogService) {
+		t.Fatal("callback must not run when dependencies are missing")
+	})
+	require.Error(t, err)
+	assert.ErrorIs(t, err, core.ErrServiceNotReady)
+	assert.ErrorIs(t, err, core.ErrServiceNotFound)
+
+	// Only LogService is missing now, so exactly one joined cause must be present.
+	core.Provide[SampleService](ctx, &sampleServiceImpl{})
+	err = core.Using2(ctx, func(s SampleService, l LogService) {
+		t.Fatal("callback must not run when a dependency is missing")
+	})
+	assert.ErrorIs(t, err, core.ErrServiceNotReady)
+	assert.ErrorIs(t, err, core.ErrServiceNotFound)
+
+	err = core.Using3(ctx, func(s SampleService, l LogService, c ConfigService) {
+		t.Fatal("callback must not run when a dependency is missing")
+	})
+	assert.ErrorIs(t, err, core.ErrServiceNotReady)
+	assert.ErrorIs(t, err, core.ErrServiceNotFound)
+}
+
 func TestContextHierarchyAndFork(t *testing.T) {
 	parent := core.NewContext(nil) // nil base context test
 	core.Provide[SampleService](parent, &sampleServiceImpl{prefix: "Parent:"})