Track preserved failed machines in MCS and MCD status

[thiyyakat]

What this PR does / why we need it:
This PR introduces 2 changes:

1. It tracks the number of preserved failed machines in the MCSs and MCDs in a new field preservedFailedReplicas added to the MCS and MCD Status.
2. It prevents the MCD from being marked as unhealthy by excluding preserved failed machines from failedMachine in the MCS and MCD Status. This will also prevent shoot reconciliation from getting stuck.

Additionally, the PR modifies the sorting logic for ActiveMachines -- if two machines are preserved, one auto-preserved and one explicitly preserved through annotation by user/operator, the sorting logic de-prioritizes explicitly preserved machines for deletion.

The usage doc for preservation has been updated with a warning regarding the behaviour of DWD when number of preserved failed machines exceeds the threshold set for DWD.

Which issue(s) this PR fixes:
Extends #1008

Special notes for your reviewer:
MCM unit and integration tests passed with changes.
The changes were manually tested out using virtual provider. Below log shows MCD status when it has 1 preserved failed replica.

status:
  conditions:
  - lastTransitionTime: "2026-04-06T07:20:38Z"
    lastUpdateTime: "2026-04-06T07:20:58Z"
    message: MachineSet "shoot--i749592--preserve-pr-preserve-pr-cpu-z1-5cc55" has
      successfully progressed.
    reason: NewMachineSetAvailable
    status: "True"
    type: Progressing
  - lastTransitionTime: "2026-04-06T11:27:16Z"
    lastUpdateTime: "2026-04-06T11:27:16Z"
    message: Deployment has minimum availability.
    reason: MinimumReplicasAvailable
    status: "True"
    type: Available
  observedGeneration: 1
  preservedFailedReplicas: 1
  replicas: 1
  unavailableReplicas: 1
  updatedReplicas: 1

Release note:

Track preserved failed machines in field `preservedFailedReplicas` in the Status of MCS and MCD, and prevent these machines from resulting in MCD being marked as unhealthy.

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign unmarshall for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aaronfern]

Docstring needs to be corrected

[r4mek]

/lgtm

[gardener-prow]

LGTM label has been added.

Details

Git tree hash: b6085293a87bf4863ad9b9ac74d7330449a41f91

[gardener-prow]

New changes are detected. LGTM label has been removed.

[gardener-prow]

PR needs rebase.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.