feat(submitqueue): Add queuepause Store contract and QueuePause entity (#862)

## Why?
Operators need to pause a queue at runtime without redeploying (see the
pause and unpause RFC). The pause state needs its own source, separate
from queue config, so each deployment can back it with whatever it uses
for runtime config.

## What?
- `entity.QueuePause`: two independent switches, `RejectLands` and
`FreezeProcessing`. The zero value means not paused.
- `queuepause.Store`: a single `Get(ctx, queue)` contract. A queue with
no entry returns the zero state, and an error means the backend could
not be read, never that the queue is unpaused.
- The mock, a README, and the new package added to the mock generation
list.

## Test Plan
`bazel build //submitqueue/extension/queuepause/...
//submitqueue/entity/...`
`make lint`
`make check-gazelle`
`make check-tidy`
CI

## Issue

Author: manjari25

Makefile

@@ -632,6 +632,7 @@ GO_GENERATE_PACKAGES := \
 	./submitqueue/extension/changeprovider/... \
 	./submitqueue/extension/conflict/... \
 	./submitqueue/extension/queueconfig/... \
+	./submitqueue/extension/queuepause/... \
 	./submitqueue/extension/speculation/... \
 	./submitqueue/extension/storage/... \
 	./submitqueue/extension/validator/... \

submitqueue/entity/BUILD.bazel

@@ -16,6 +16,7 @@ go_library(
         "path_build.go",
         "queue_batch_state.go",
         "queue_config.go",
+        "queue_pause.go",
         "request.go",
         "request_batch.go",
         "request_history.go",

submitqueue/entity/queue_pause.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2026 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 entity
+
+// QueuePause is a point-in-time snapshot of the pause switches of a single
+// queue. The two switches are independent, and the zero value means the queue
+// is not paused.
+type QueuePause struct {
+	// RejectLands is true while the queue refuses new land requests.
+	RejectLands bool
+
+	// FreezeProcessing is true while the queue's accepted requests are not advanced.
+	FreezeProcessing bool
+
+	// Reason is the operator's free-text explanation for the pause. Empty when none was given.
+	Reason string
+}

submitqueue/extension/queuepause/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["queuepause.go"],
+    importpath = "github.com/uber/submitqueue/submitqueue/extension/queuepause",
+    visibility = ["//visibility:public"],
+    deps = ["//submitqueue/entity:go_default_library"],
+)

submitqueue/extension/queuepause/README.md

@@ -0,0 +1,15 @@
+# Queue Pause Extension
+
+Vendor-agnostic source of per-queue pause state. See [the pause and unpause RFC](../../../doc/rfc/submitqueue/queue-pause-and-unpause.md) for the design.
+
+## Interfaces
+
+### Store
+
+Returns the pause state of a queue by name. The state is two independent switches: reject new land requests, and freeze processing of accepted requests. A queue with no entry is not paused, and an error means the backend could not be read.
+
+The store only reports state. The gateway and orchestrator enforce it, so every deployment runs the same rejection and freezing logic and only the source of the state differs.
+
+## Entities
+
+`QueuePause` lives in `submitqueue/entity/queue_pause.go`.

submitqueue/extension/queuepause/mock/BUILD.bazel

@@ -0,0 +1,12 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["queuepause_mock.go"],
+    importpath = "github.com/uber/submitqueue/submitqueue/extension/queuepause/mock",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//submitqueue/entity:go_default_library",
+        "@org_uber_go_mock//gomock:go_default_library",
+    ],
+)

submitqueue/extension/queuepause/mock/queuepause_mock.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2026 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 queuepause
+
+//go:generate mockgen -source=queuepause.go -destination=mock/queuepause_mock.go -package=mock
+
+import (
+	"context"
+
+	"github.com/uber/submitqueue/submitqueue/entity"
+)
+
+// Store is the source of per-queue pause state. It reports the switches and
+// decides nothing; enforcement lives in the services that resolve it.
+type Store interface {
+	// Get returns the current pause state of a queue. A queue with no entry
+	// returns the zero state (not paused) and no error. A non-nil error means
+	// the backend could not be read, not that the queue is unpaused; a caller
+	// that enforces a switch must not fail open on it. Implementations must be
+	// safe for concurrent use.
+	Get(ctx context.Context, queue string) (entity.QueuePause, error)
+}