WIP!: maintnofications for cluster

[ndyakov]

This PR is still work in progress.

TODO:

• fix ClusterOptions missing DialerRetries&DialerRetryTimeout #3594

[Copilot]

Pull Request Overview

This PR adds support for SMIGRATING and SMIGRATED notifications to enable cluster slot migration handling in ClusterClient. These notifications allow the client to adapt to cluster topology changes during slot migrations.

• Implements SMIGRATING handler to apply relaxed timeouts during slot migration
• Implements SMIGRATED handler to trigger cluster state reload when migration completes
• Adds callback mechanism for node clients to notify parent ClusterClient of state changes

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
osscluster.go	Registers SMIGRATED callback on node clients to trigger cluster state reload
maintnotifications/smigrating_test.go	Comprehensive test coverage for SMIGRATING and SMIGRATED notification handlers
maintnotifications/push_notification_handler.go	Implements handleSMigrating and handleSMigrated notification handlers
maintnotifications/manager_test.go	Updates test to verify new notification types are registered
maintnotifications/manager.go	Adds cluster state reload callback mechanism and new notification constants
maintnotifications/README.md	Documents cluster support for SMIGRATING and SMIGRATED notifications
internal/maintnotifications/logs/log_messages.go	Adds logging functions for cluster notification handling
cluster_smigrating_test.go	Integration tests verifying callback setup and notification handling in cluster context

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[Copilot]

The TODO suggests uncertainty about whether relaxed timeouts should be cleared after SMIGRATED. This should be resolved before merging - either clear the timeout (if conn is available) or document why it's intentionally not cleared. Consider the parallel behavior in handleMigrated which clears timeouts on line 290.

Suggested change

	// TODO: Should we also clear the relaxed timeout here (like MIGRATED does)?
	// Currently we only trigger state reload, but the timeout stays relaxed
	// Clear the relaxed timeout on the connection, if available, to match MIGRATED behavior.
	if handlerCtx.Conn != nil {
	handlerCtx.Conn.ClearRelaxedTimeout()
	}