SYS-4189 Fix nonce for self pay transactions

[nahuseyoum]

Proposed changes

This PR fixes a bug where self pay proxy transactions were failing due to a bad proxy signature. New tests are also added to guard against this for the future.

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• Verify First, Write Last: In Substrate development, it is important that you always ensure preconditions are met and return errors at the beginning. After these checks have completed, then you may begin the function's computation.

Further comments

[micaelffrancoAV]

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