🐛 Fix machine loop block when backing node does not exist

[viragvoros]

What this PR does / why we need it:
This PR fixes a bug in the machine preservation logic (manageMachinePreservation). Currently, the controller returns an error if the backing Node object does not exist yet in the cluster.
When a machine is newly created or replacing an old instance, it takes time for the node to register. Returning an error here completely stops the machine's main update loop. This prevents the machine from changing its status phase, which causes downstream components to miss other checks.
By ignoring the NotFound error, the controller can assume there are no node annotations yet and continue updating the machine normally.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
I added a new test case to the unit test table. It checks that when a node is missing from the cluster, the controller ignores the error and returns a normal LongRetry instead of failing.

Release note:

bugfix operator
Fixed a bug where a missing Node object stopped the machine controller loop from running, causing machine status phase updates to get stuck.

[gardener-prow]

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[gagan16k]

Was this guard intended to be on uncordonNodeIfCordoned(), as getNodePreserveAnnotation()already has a check for IsNotFound errors in the function?

We are aware of that particular case, and are working on a fix for it. If this PR is intended to solve the same bug, we would prefer to close this in favor of the other one.
Refer to #1110 for tracking
Thanks!

[Kumm-Kai]

@gagan16k

If this PR is intended to solve the same bug, we would prefer to close this in favor of the other one.

What/where is the "other one" PR?

[thiyyakat]

@Kumm-Kai : The PR was raised only recently: #1111. @gagan16k was unable to provide a link to it earlier.