rpk: add high disk usage report on 'cluster health'

[r-vasquez]

This was introduced in 5b5286f and already released in 25.3.5

Solves INC-1051

Example

$ rpk cluster health
CLUSTER HEALTH OVERVIEW
=======================
Healthy:                          false
Unhealthy reasons:                [high_disk_usage_nodes]
Controller ID:                    0
All nodes:                        [0]
Nodes down:                       []
Nodes in recovery mode:           []
Nodes with high disk usage:       [0]
Leaderless partitions (0):        []
Under-replicated partitions (0):  []
Cluster UUID:                     1e0e7ba3-b3ca-4e6b-a8ca-453170d6b0af

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

Features

• rpk cluster health: high_disk_usage_nodes is now reported.

[Copilot]

Pull request overview

This PR adds support for displaying nodes with high disk usage in the rpk cluster health command output. The feature was previously added in commit 5b5286f and released in version 25.3.5, and this change surfaces that information in the CLI.

Changes:

• Added HighDiskUsageNodes field to the health response structure
• Updated the health overview output to display nodes with high disk usage
• Updated protobuf and dependency versions to support the new field

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/go/rpk/pkg/cli/cluster/health.go	Added HighDiskUsageNodes field to struct and print logic to display nodes with high disk usage
src/go/rpk/go.mod	Updated dependencies including protobuf definitions and rpadmin library to versions that support high disk usage reporting
buf.gen.yaml	Updated core module reference to newer version containing high disk usage definitions