Enable the Lint/UnreachableCode cop and remove its exclusion from .rubocop_todo.yml.

Two offenses:

• lib/mongo/protocol/serializers.rb - Sections::PayloadOne.deserialize is an explicit stub that raises NotImplementedError; the code after the raise is unreachable dead code (an abandoned implementation). Remove it, and widen the signature to (buffer, _options = {}) to match its call site.
• spec/runners/unified/test.rb - the isTimeoutError branch has a stray `raise e` before the intended Error::ErrorMismatch raise, making the latter unreachable. Remove the stray raise.

Part of RUBY-3815.