[TRT RTX EP] Fix bug for generating the correct subgraph in GetCapability (#26132)
### Description
In current TRT RTX EP/ TRT EP implementation, when constructing the
`IndexedSubGraph`, for some cases, it will include the node's unused
output as the SubGraph's output. So, it will return the incorrect
`IndexedSubGraph` from its GetCapability to ORT.
Add the logic to prevent adding the unused node's output.
With this fix, we can avoid generating the incorrect EPContext model
where the EPContext node has unused output.