To my recollection, i have never seen any info on TF35000's first firing or the start of a prototype building. What we were told is that some early tests will be conducted in 2026. That doesn't mean a prototype. It will be a test article for early testing before committing to a prototype. So it's a part of the engine that will be tested on a specialized rig. It could belong to the compressor, combustion chamber, turbine section and what not.
When they start to build the prototype, it will take 1.5-2 years before it's completed and perform its first firing. We will be shown its full-scale mock-up before the prototype. But that requires an advanced stage in development. The program is still in preliminary-design phase. I for one would feel ecstatic if they could fire the engine first time by 2028.