refactor(queue/mysql): clean up logging, metrics naming, and error handling

- Add queue_mysql_ prefix to all logger Named() and metrics SubScope()
  calls for consistent scoping across stores
- Rename metrics field to scope, use metrics to refer to core/metrics package
- Remove redundant pre-operation debug logs (e.g. "inserting messages")
- Elevate "message not found for DLQ move" from Debug to Warn
- Remove redundant "subscription created" debug log
- Replace manual error tagging with core/metrics framework (Begin/Complete)
- Remove unused constants (tagErrorType, errorBeginTx, errorCommit)

Author: behinddwalls

extension/queue/mysql/constants.go

@@ -17,16 +17,9 @@ package mysql
 // 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"
 )