refactor(storage): fold ChangeStore into storage, drop dead store

## Summary

### Why?

Two "change"-related stores were in the wrong shape. `ChangeStore` — the
real, used store that records per-URI claims for in-flight requests and
backs `start`'s URI claiming and `validate`'s overlap detection — lived
as its own top-level extension and was injected into controllers as a
separate dependency, bypassing the `storage.Storage` aggregator that owns
every other entity store. Meanwhile `ChangeProviderStore` (exposed via
`Storage.GetChangeProviderStore()`, with a mysql impl, mock, and schema)
was dead code: no controller ever called it, and its `entity.ChangeProvider`
was orphaned alongside it.

### What?

- Move `ChangeStore` into `package storage`: the interface, mysql impl,
  and `change.sql` schema now live under `extension/storage[/mysql]`, and
  `ChangeStore` is a first-class member of the `Storage` aggregator via
  `GetChangeStore()` — matching every other store.
- `start` and `validate` controllers drop their separate `changeStore`
  constructor param and read `store.GetChangeStore()`; the example
  orchestrator no longer constructs/injects it separately.
- Delete the dead `ChangeProviderStore` (interface, mysql, mock, schema),
  `GetChangeProviderStore()`, and the orphaned `entity/change_provider.go`.
- Fold the standalone changestore integration suite into the shared
  `StorageContractSuite` (driven through `GetChangeStore()`); e2e drops the
  now-redundant changestore schema apply.

## Test Plan

- ✅ `make build`, `make test` (start/validate controllers pass against the
  storage-package mock)
- ✅ `make lint`, `make check-gazelle`, `make check-mocks`, `make check-tidy`
  (no drift; `go.mod` / `MODULE.bazel` unchanged)
- ✅ `make integration-test` (storage mysql suite now exercises the change
  store via `GetChangeStore()`)
- ✅ `make e2e-test` (full land→validate flow: URI claim + overlap detection,
  `change` table applied from the storage schema dir)

Author: behinddwalls

Makefile

@@ -334,7 +334,7 @@ local-stovepipe-gateway-start: build-stovepipe-gateway-linux ## Start Stovepipe
 
 mocks: ## Generate mock files using mockgen
 	@echo "Generating mocks..."
-	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./submitqueue/extension/changestore/... ./extension/counter/... ./extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/pusher/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/core/consumer/...
+	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./extension/counter/... ./extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/pusher/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/core/consumer/...
 	@echo "Mocks generated successfully!"
 
 proto: ## Generate protobuf files from .proto definitions

example/submitqueue/orchestrator/server/BUILD.bazel

@@ -24,8 +24,6 @@ go_library(
         "//submitqueue/extension/buildrunner/noop",
         "//submitqueue/extension/changeprovider",
         "//submitqueue/extension/changeprovider/github",
-        "//submitqueue/extension/changestore",
-        "//submitqueue/extension/changestore/mysql",
         "//submitqueue/extension/conflict",
         "//submitqueue/extension/conflict/all",
         "//submitqueue/extension/mergechecker",

example/submitqueue/orchestrator/server/main.go

@@ -43,8 +43,6 @@ import (
 	buildnoop "github.com/uber/submitqueue/submitqueue/extension/buildrunner/noop"
 	"github.com/uber/submitqueue/submitqueue/extension/changeprovider"
 	githubprovider "github.com/uber/submitqueue/submitqueue/extension/changeprovider/github"
-	"github.com/uber/submitqueue/submitqueue/extension/changestore"
-	mysqlchangestore "github.com/uber/submitqueue/submitqueue/extension/changestore/mysql"
 	"github.com/uber/submitqueue/submitqueue/extension/conflict"
 	"github.com/uber/submitqueue/submitqueue/extension/conflict/all"
 	"github.com/uber/submitqueue/submitqueue/extension/mergechecker"
@@ -164,8 +162,6 @@ func run() error {
 		return fmt.Errorf("failed to create storage: %w", err)
 	}
 
-	changeStore := mysqlchangestore.NewChangeStore(appDB, scope.SubScope("changestore"))
-
 	// Open queue database connection
 	// Docker Compose healthchecks ensure MySQL is ready before service starts
 	queueDSN := os.Getenv("QUEUE_MYSQL_DSN")
@@ -234,7 +230,7 @@ func run() error {
 	br := buildnoop.New()
 
 	// Register controllers
-	if err := registerControllers(c, logger.Sugar(), scope, registry, mc, cp, psh, br, cnt, store, changeStore); err != nil {
+	if err := registerControllers(c, logger.Sugar(), scope, registry, mc, cp, psh, br, cnt, store); err != nil {
 		return err
 	}
 
@@ -460,12 +456,11 @@ type conflictFactory struct{ impl conflict.Analyzer }
 
 func (f conflictFactory) For(conflict.Config) (conflict.Analyzer, error) { return f.impl, nil }
 
-func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope tally.Scope, registry consumer.TopicRegistry, mc mergechecker.MergeChecker, cp changeprovider.ChangeProvider, psh pusher.Pusher, br buildrunner.BuildRunner, cnt counter.Counter, store storage.Storage, changeStore changestore.ChangeStore) error {
+func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope tally.Scope, registry consumer.TopicRegistry, mc mergechecker.MergeChecker, cp changeprovider.ChangeProvider, psh pusher.Pusher, br buildrunner.BuildRunner, cnt counter.Counter, store storage.Storage) error {
 	requestController := start.NewController(
 		logger,
 		scope,
 		store,
-		changeStore,
 		registry,
 		consumer.TopicKeyStart,
 		"orchestrator-start",
@@ -490,7 +485,6 @@ func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope t
 		logger,
 		scope,
 		store,
-		changeStore,
 		registry,
 		mergeCheckerFactory{impl: mc},
 		changeProviderFactory{impl: cp},

submitqueue/entity/BUILD.bazel

@@ -7,7 +7,6 @@ go_library(
         "batch_dependent.go",
         "build.go",
         "cancel_request.go",
-        "change_provider.go",
         "change_record.go",
         "land_request.go",
         "queue_config.go",