🐛 Enforce rollout strategy/type consistency and warn on disabled topology MHC overrides

[miltalex]

What this PR does / why we need it:

Adds missing rollout strategy validation parity for MachineDeployment, Cluster.spec.topology, and ClusterClass (reject rollingUpdate fields when strategy type is OnDelete), with ratcheting so unchanged pre-existing invalid values are still allowed on update. It also changes disabled topology MHC override handling to emit webhook warnings (not errors) so users can temporarily disable MHC for troubleshooting without removing config.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #8721

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[sbueringer]

It also changes disabled topology MHC override handling to emit webhook warnings (not errors) so users can temporarily disable MHC for troubleshooting without removing config.

I don't think we should allow invalid configuration just because it's convenient in cases like this

[miltalex]

I don't think we should allow invalid configuration just because it's convenient in cases like this

Sharing the context from the slack conversation:

Keeping MHC fields while disabling MHC is already possible today (before this PR).

Before / current behavior:
  - spec.topology.*.healthCheck.enabled=false with checks / remediation still set:
      - Allowed
      - No warning
      - Fields are effectively ignored while disabled, but remain in the object
  - OnDelete strategy + rollingUpdate fields:
      - Allowed (no webhook validation catches the mismatch)

PR is introducing:
  - spec.topology.*.healthCheck.enabled=false with checks / remediation still set:
      - Still allowed (same as today)
      - New webhook warning is returned saying those fields are ignored while disabled
  - OnDelete strategy + rollingUpdate fields:
      - Rejected when newly introduced (or changed while invalid)
      - Unchanged pre-existing invalid values are still allowed on update (so users are not blocked following your comment)
      - Fixing an existing invalid config is allowed

[sbueringer]

/ok-to-test
(not sure when I'll find time for review)