fix(coderd/provisionerdserver): clarify duplicate preset errors (#28530)

> 🤖 This PR was written by Coder Agents on behalf of Jake Howell.

Template imports with duplicate preset names currently expose the
underlying PostgreSQL unique constraint error.

Translate that constraint violation into a clear validation message that
identifies the duplicate preset name.

<img width="1110" height="462" alt="image"
src="https://github.com/user-attachments/assets/dc7b86a3-8f85-4694-a0be-aea3d0b53360"
/>


Closes https://github.com/coder/coder/issues/19270

coderd/provisionerdserver/provisionerdserver.go

@@ -2979,6 +2979,9 @@ func InsertWorkspacePresetAndParameters(ctx context.Context, db database.Store,
 
 		return nil
 	}, nil)
+	if database.IsUniqueViolation(err, database.UniqueIndexUniquePresetName) {
+		return xerrors.Errorf("duplicate preset name, must be unique per template: %q", protoPreset.Name)
+	}
 	if err != nil {
 		return xerrors.Errorf("insert preset and parameters: %w", err)
 	}

coderd/provisionerdserver/provisionerdserver_test.go

@@ -4009,6 +4009,36 @@ func TestInsertWorkspacePresetsAndParameters(t *testing.T) {
 			}
 		})
 	}
+
+	t.Run("duplicate preset names", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitLong)
+		logger := testutil.Logger(t)
+		db, ps := dbtestutil.NewDB(t)
+		org := dbgen.Organization(t, db, database.Organization{})
+		user := dbgen.User(t, db, database.User{})
+		job := dbgen.ProvisionerJob(t, db, ps, database.ProvisionerJob{
+			Type:           database.ProvisionerJobTypeWorkspaceBuild,
+			OrganizationID: org.ID,
+		})
+		templateVersion := dbgen.TemplateVersion(t, db, database.TemplateVersion{
+			JobID:          job.ID,
+			OrganizationID: org.ID,
+			CreatedBy:      user.ID,
+		})
+
+		err := provisionerdserver.InsertWorkspacePresetsAndParameters(
+			ctx,
+			logger,
+			db,
+			job.ID,
+			templateVersion.ID,
+			[]*sdkproto.Preset{{Name: "daily-driver"}, {Name: "daily-driver"}},
+			time.Now(),
+		)
+		require.ErrorContains(t, err, `duplicate preset name, must be unique per template: "daily-driver"`)
+	})
 }
 
 func TestInsertWorkspaceResource(t *testing.T) {