feat(queue/sql): add data access layer stores

## Why?
Need data access layer to abstract database operations for message storage, offset tracking, and partition leasing.

## What?
- MessageStore: insert, fetch, delete, DLQ, and visibility timeout operations
- OffsetStore: consumer offset tracking with atomic ack
- PartitionLeaseStore: distributed partition leasing with automatic discovery
- Comprehensive test coverage using in-memory MySQL

## Test Plan
- All store operations tested (insert, fetch, ack, lease)
- Atomic operations verified (visibility timeout, offset updates)
- Partition discovery and lease acquisition tested
- DLQ operations validated

Author: behinddwalls

extensions/queue/sql/BUILD.bazel

@@ -2,16 +2,44 @@ load("@rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "sql",
-    srcs = ["config.go"],
+    srcs = [
+        "config.go",
+        "constants.go",
+        "message_store.go",
+        "offset_store.go",
+        "partition_lease_store.go",
+        "test_helpers.go",
+    ],
     importpath = "github.com/uber/submitqueue/extensions/queue/sql",
     visibility = ["//visibility:public"],
+    deps = [
+        "//entities/queue",
+        "@com_github_dolthub_go_mysql_server//:go-mysql-server",
+        "@com_github_dolthub_go_mysql_server//memory",
+        "@com_github_dolthub_go_mysql_server//server",
+        "@com_github_dolthub_go_mysql_server//sql",
+        "@com_github_dolthub_vitess//go/mysql",
+        "@com_github_go_sql_driver_mysql//:mysql",
+        "@com_github_stretchr_testify//require",
+        "@com_github_uber_go_tally_v4//:tally",
+        "@org_uber_go_zap//:zap",
+        "@org_uber_go_zap//zaptest",
+    ],
 )
 
 go_test(
     name = "sql_test",
-    srcs = ["config_test.go"],
+    srcs = [
+        "config_test.go",
+        "message_store_test.go",
+        "offset_store_test.go",
+        "partition_lease_store_test.go",
+    ],
+    data = ["//schema/queue/mysql:schema.sql"],
     embed = [":sql"],
     deps = [
+        "//entities/queue",
+        "@com_github_go_sql_driver_mysql//:mysql",
         "@com_github_stretchr_testify//assert",
         "@com_github_stretchr_testify//require",
     ],

extensions/queue/sql/constants.go

@@ -0,0 +1,18 @@
+package sql
+
+// Common constants for frequently repeated strings across stores
+
+const (
+	// Tag key (used in every Tagged() call)
+	tagErrorType = "error_type"
+
+	// Common log field names (used extensively across all stores)
+	logTopic        = "topic"
+	logPartitionKey = "partition_key"
+	logMessageID    = "message_id"
+	logError        = "error"
+
+	// Error types used across multiple methods/stores
+	errorBeginTx = "begin_transaction"
+	errorCommit  = "commit"
+)