/validation MUST NOT complain about unresolvable process parameters

reported by a user: trying to validate a parameterized UDP raises [{'code': 'ProcessParameterRequired', 'message': "Process 'n/a' parameter '...' is required."} issues, while openEO API explicitly states:

The validation MUST NOT throw an error for unresolvable process parameters.

reproduction example

import openeo
from openeo.api.process import Parameter
from openeo.processes import add
from openeo.rest.udp import build_process_dict

con = openeo.connect("openeo.vito.be").authenticate_oidc()

x = Parameter.number("x", description="x")
add1 = add(x, 1)
spec = build_process_dict(process_id="add1", process_graph=add1, parameters=[x])

resp = con.post("/validation", json=spec)
print(resp.json())

->

{'errors': [{'code': 'ProcessParameterRequired', 'message': "Process 'n/a' parameter 'x' is required."}]}

Start with the reproduced POST request to the /validation endpoint and the parameterized UDP built by build_process_dict. Trace where ProcessParameterRequired is produced for unresolved parameters, then verify that validation accepts the example without that error while retaining relevant validation errors.