Is it permissible for me to search for and accept better job opportunities during the notice period after accepting a job offer from a company, knowing that job offers are not legally binding in my country until final onboarding?

The lease contract is a binding contract that cannot be rescinded except with the consent of both parties, and jobs fall under this category. The fundamental principle of a lease contract, according to the majority of scholars, is its binding nature, so neither party has the right to unilaterally terminate the contract. If the agreement with Company B was that the work would commence after two months, and the final agreement would be upon joining, then you are free to look for another company and not work with Company B; this is because the contract is not yet binding, and what exists between you is a non-binding promise.