[EP ABI] Node_GetAttrByName returns ORT_NOT_FOUND with non-existing attr name (#25565)
### Description
Updates `Node_GetAttributeByName` to return an error status with code
`ORT_NOT_FOUND` and set the `attribute` output parameter to `NULL` when
called with a non-existing attribute name.
Why? Currently, a caller has to do string comparison of the `OrtStatus`
error message to determine if the attribute does not exist or if another
error occurred. This can be somewhat cumbersome. With this change, the
caller can just check the error code.
### Motivation and Context
Make it easier to use `Node_GetAttributeByName`.