feat(buildrunner/noop): add pass-through BuildRunner stub

## Summary

### Why?

Wiring tests for the orchestrator's build stage need a `BuildRunner`
implementation, but real backends (BuildKite, Jenkins, etc.) won't land
until later. A noop that immediately reports success is also useful as
the local default in `make local-start` and as a best-case baseline
where every build passes.

### What?

Adds `extension/buildrunner/noop` — a `BuildRunner` that does no real
work. Every `Trigger` returns a unique `noop-<n>` build ID with no
error; every `Status` returns `BuildStatusSucceeded` with no metadata;
`Cancel` is a no-op. The atomic counter keeps IDs unique under
concurrent use.

## Test Plan

- ✅ `make test` — 36 unit tests pass, including the new
  `extension/buildrunner/noop:noop_test` (Trigger uniqueness, Status,
  Cancel, interface satisfaction).
- ✅ `make fmt lint check-tidy check-gazelle check-mocks` — clean.
- ✅ `make build` — all targets compile.

Author: behinddwalls

extension/buildrunner/noop/BUILD.bazel

@@ -0,0 +1,24 @@
+load("@rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "noop",
+    srcs = ["noop.go"],
+    importpath = "github.com/uber/submitqueue/extension/buildrunner/noop",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//entity",
+        "//extension/buildrunner",
+    ],
+)
+
+go_test(
+    name = "noop_test",
+    srcs = ["noop_test.go"],
+    embed = [":noop"],
+    deps = [
+        "//entity",
+        "//extension/buildrunner",
+        "@com_github_stretchr_testify//assert",
+        "@com_github_stretchr_testify//require",
+    ],
+)

extension/buildrunner/noop/noop.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package noop provides a buildrunner.BuildRunner that performs no real
+// work: every triggered build immediately succeeds. It is intended as a
+// stub for wiring tests and as a best-case baseline where every build
+// passes.
+package noop
+
+import (
+	"context"
+	"fmt"
+	"sync/atomic"
+
+	"github.com/uber/submitqueue/entity"
+	"github.com/uber/submitqueue/extension/buildrunner"
+)
+
+// runner is a buildrunner.BuildRunner that does no real work and reports
+// every build as immediately succeeded. The atomic counter hands out
+// unique build IDs and makes the type safe for concurrent use.
+type runner struct {
+	counter atomic.Uint64
+}
+
+// New returns a buildrunner.BuildRunner that performs no real work.
+func New() buildrunner.BuildRunner {
+	return &runner{}
+}
+
+// Trigger returns a unique build ID without contacting any runner.
+// Inputs are ignored.
+func (r *runner) Trigger(_ context.Context, _ string, _ []entity.Change, _ []entity.Change, _ entity.BuildMetadata) (entity.BuildID, error) {
+	return entity.BuildID{ID: fmt.Sprintf("noop-%d", r.counter.Add(1))}, nil
+}
+
+// Status always reports BuildStatusSucceeded with no metadata.
+func (r *runner) Status(_ context.Context, _ entity.BuildID) (entity.BuildStatus, entity.BuildMetadata, error) {
+	return entity.BuildStatusSucceeded, nil, nil
+}
+
+// Cancel is a no-op.
+func (r *runner) Cancel(_ context.Context, _ entity.BuildID) error {
+	return nil
+}

extension/buildrunner/noop/noop_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package noop
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/uber/submitqueue/entity"
+	"github.com/uber/submitqueue/extension/buildrunner"
+)
+
+func TestNew_ImplementsInterface(t *testing.T) {
+	var _ buildrunner.BuildRunner = New()
+}
+
+func TestRunner_Trigger(t *testing.T) {
+	r := New()
+	ctx := context.Background()
+
+	id1, err := r.Trigger(ctx, "queueA",
+		[]entity.Change{{URIs: []string{"github://owner/repo/pull/1"}}},
+		[]entity.Change{{URIs: []string{"github://owner/repo/pull/2"}}},
+		entity.BuildMetadata{"requester": "alice"},
+	)
+	require.NoError(t, err)
+	assert.NotEmpty(t, id1.ID)
+
+	// IDs are unique across calls, even with empty inputs.
+	id2, err := r.Trigger(ctx, "queueA", nil, nil, nil)
+	require.NoError(t, err)
+	assert.NotEqual(t, id1, id2)
+}
+
+func TestRunner_Status(t *testing.T) {
+	r := New()
+
+	status, meta, err := r.Status(context.Background(), entity.BuildID{ID: "any-id"})
+	require.NoError(t, err)
+	assert.Equal(t, entity.BuildStatusSucceeded, status)
+	assert.Empty(t, meta)
+}
+
+func TestRunner_Cancel(t *testing.T) {
+	r := New()
+	assert.NoError(t, r.Cancel(context.Background(), entity.BuildID{ID: "any-id"}))
+}