fix(cp): fall back to __opts__ when __file_client__ is not packed

[dwoz]

What does this PR do?

salt.modules.cp._client() did:

if __file_client__:
    return __file_client__.value()
return salt.fileclient.get_file_client(__opts__.value())

__file_client__ is a NamedLoaderContext. When the loader executing the cp
module has not packed a __file_client__ (any loader other than
minion_mods that omits a file client — e.g. the resource module loader),
evaluating if __file_client__: raises LoaderError rather than returning a
falsey value. That raise short-circuits the intended __opts__ fallback, so
cp.cache_file — and therefore every salt:// fetch a state performs —
fails with KeyError: '__file_client__'.

This guards the context lookup and falls back to building a file client from
__opts__ when the context is missing/None.

What issues does this PR fix or reference?

Fixes #69734

Previous Behavior

cp._client() raised LoaderError/KeyError: '__file_client__' on any loader
that did not pack a file client, breaking salt:// fetches.

New Behavior

cp._client() returns the packed file client when present, otherwise builds one
from __opts__ — no exception.

Tests written?

Yes — tests/pytests/unit/modules/test_cp.py:

• test__client_returns_packed_file_client
• test__client_falls_back_when_file_client_not_packed

Commits signed with GPG?

No