fix(Certora specs): ensure prover runs rules on currentContract

[0x-r4bbit]

Since we're implementing rules for StakeManager migrations, we need multiple instances inside the certora specs.

This results in the prover trying to run rules on the other StakeManager instance as well, which isn't always desired, as it causes some rules to fail, even though they'd pass if they'd be executed only on the currentContract.

This commit makes the filter condition for relevant rules stronger, such that the prover will not run them on the newStakeManager contract instance.

Checklist

Ensure you completed all of the steps below before submitting your pull request:

• Added natspec comments?
• Ran forge snapshot?
• Ran pnpm gas-report?
• Ran pnpm lint?
• Ran forge test?
• Ran pnpm verify?

[3esmit]

This makes sense, caused by the new modifier blocking calls while in migration:

    /**
     * @notice Only callable when migration is not initialized.
     */
    modifier noMigration() {
        if (address(migration) != address(0)) {
            revert StakeManager__PendingMigration();
        }
        _;
    }

As the prover would havoc the address storage of migration StakeManager public migration, this would impact the call of function startMigration(StakeManager _migration) external, which after refactor uses this new modifier noMigration .