Proto: abstract `Full_staking_balance_repr`

Context

First part of !10455 (merged)

More than just a refactoring:

• the minimal stake should be better re-checked against the stake_repr after applying limits,
• when AI is activated, the minimal stake will be re-checked against the total stake after the edge of staking over delegating is applied.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
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