group_by: partition_by does not support inline func expressions

[dmpetrov]

Description

When passing an inline func.* expression directly to partition_by in group_by, DataChain silently derives an internal column name and then fails with a SignalResolvingError because that column doesn't exist in the schema yet.

Steps to reproduce

import datachain as dc
from datachain import C, func

(
    dc.read_storage("gs://datachain-demo/", anon=True)
    .group_by(
        count=func.count(),
        total_size=func.sum(C("file.size")),
        partition_by=func.path.parent(C("file.path")),  # inline expression
    )
    .show()
)

Error

datachain.lib.signal_schema.SignalResolvingError: cannot resolve signal name 'file__path__parent': is not found

Expected behavior

Either:

• Inline expressions in partition_by are evaluated automatically (like they are in mutate), or
• A clear error message explaining that partition_by requires a pre-existing column name, not an expression

Workaround

Materialize the column with .mutate() first:

(
    dc.read_storage("gs://datachain-demo/", anon=True)
    .mutate(parent=func.path.parent(C("file.path")))
    .group_by(
        count=func.count(),
        total_size=func.sum(C("file.size")),
        partition_by="parent",
    )
    .show()
)