The fixture brackets should be included with the fixtures, but supporting means will be on you, unless the fixtures are specified with aircraft cable hangers, etc. . ., and that will typically be listed in the fixture schedule.
Dimmers are probably on you, but you would need to clarify. I typically will get a BOM on the fixture package if the owner is supplying, and I clarify everything in my proposal. I know most of the times when I send an RFQ for fixtures, it will come back with dimmers included even if I don't have them in my take-off; and that's on larger jobs that several people are bidding.
Anyway, if you don't get a BOM while bidding, you should at least get one before you sign a contract. I will ask for this on anything the owner is providing; fixtures, gear, etc. . . .
As for responsibility, the specs will typically say "the contractor" is responsible for receiving them, storing them, etc. . . ; check for that, price accordingly. I've seen specs also that require you to provide a certain amount of warranty labor for fixtures provided by others.
To me, it doesn't matter who buys what; just clearly word your proposal and charge whatever dollars you need to make it worth your while.