I am sorry to hear about the challenges and frustrations that you are going through.
While bacterial urinary tract infections (UTI) are quite common, yeast UTIs are highly uncommon. While Candida species can infect the urinary tract, it is unlikely unless there is concurrent disease or immune compromise of some sort. The risk of yeast entering the blood stream and translocating into a systemic or new local infection is even more rare without concurrent disease.
There are a couple possibilities that I would be considering:
1) There is a true yeast UTI. If this is the case, then the further tests that your veterinarian is recommending may be worthwhile to rule out concurrent diseases. Antifungals like fluconazole can work quite well. In humans with candida UTIs, fluconazole has been reported to work as well as amphotericin. Fluconazole is a lot safer of course than amphotericin. Of course, if severe liver issues are present, then fluconazole may not be suitable. Both long and short term fluconazole use is typically well tolerated by most healthy dogs.
2) The yeast was a spurious finding. Retesting of the urine from a cystocentesis (direct sampling from the bladder) might be worthwhile.
Regardless, the last thing that anyone would ever want is to proceed with a major orthopedic procedure and then have a complication. Urine cultures are typically standard before this type of surgery and it is standard to follow up any abnormal results. Again, yeast UTIs are very uncommon in dogs and may often be associated with underlying disease. Whether this is an extremely rare isolated yeast UTI, an uncommon yeast UTI with underlying disease or immune compromise, or a spurious laboratory finding, a methodical and careful workup is recommended.
Best wishes and I hope this helps.