[FLINK-35292] Set dummy savepoint path during last-state upgrade

[gyfora]

What is the purpose of the change

Currently the operator always sets the savepoint path even if last-state (HA metadata) must be used.

This can be misleading to users as the set savepoint path normally should never take effect and can actually lead to incorrect state restored if the HA metadata is deleted by the user at the wrong moment.

To avoid this we can set an explicit dummy savepoint path which will prevent restoring from it accidentally.

Brief change log

• Set dummy savepoint path during last-state upgrade
• Add/update tests

Verifying this change

Unit tests + manual verification

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: yes

Documentation

• Does this pull request introduce a new feature? no