Update simulation tests for SN, Val 1:1 invariant

[mateeullahmalik]

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[roomote]

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Review complete. No issues found.

The PR correctly updates simulation tests to handle the SuperNode/Validator 1:1 invariant by:

• Adding GetSuperNodeByAccount to the SupernodeKeeper interface for looking up supernodes by account
• Adding validation in SimulateMsgUpdateSupernode to ensure new accounts are not already associated with another validator
• Adding validation in registerSupernode helper to check account availability before registration
• Adding a new negative test SimulateMsgUpdateSupernodeInvalidAccount that verifies the system properly rejects attempts to use a conflicting supernode account

Previous reviews

• 646b368: Review #1

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