Checkpoint_conversion: import GCS via gcloud_stub for decoupled mode

[gulsumgudukbay]

Description

src/maxtext/checkpoint_conversion/utils/utils.py does a top-level from google.cloud.storage import Client, transfer_manager. When tests run with DECOUPLE_GCLOUD=TRUE (and google-cloud-storage is not installed in that environment), pytest collection fails on this file with ModuleNotFoundError before tests/conftest.py can apply the decoupled marker filter to deselect the test, so the whole collection aborts.
This PR routes the import through the existing gcloud_stub.gcs_storage() helper (same pattern already used by src/maxtext/utils/gcs_utils.py), so the module imports cleanly in decoupled mode while behaving identically in normal mode.

• gcloud_stub.gcs_storage(): also imports and attaches the transfer_manager submodule (it isn't auto-imported by from google.cloud import storage). _gcs_stubs() is extended with a matching no-op transfer_manager stub exposing upload_many_from_filenames.
• checkpoint_conversion/utils/utils.py: drops the direct from google.cloud.storage import ... and binds Client / transfer_manager at module top via gcs_storage(). No call sites change, the public symbol surface (Client, transfer_manager) is preserved.

(cherry picked from commit 698624b)

Tests

• Module imports cleanly in a venv without google-cloud-storage: python3 -c 'from maxtext.checkpoint_conversion.utils import utils; print(utils.Client, utils.transfer_manager)'.
• pytest --collect-only -m 'not cpu_only and not tpu_only and not post_training and decoupled and not scheduled_only' --ignore=tests/post_training collects 719/1064 tests with exit 0 (was 1 collection error before the fix).
• In a venv where google-cloud-storage is installed, gcs_storage() returns the real module with both .Client and .transfer_manager attributes.
• CI green on this branch.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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