Build: Bump gcsfs to 2026.6.0

[kevinjqliu]

Summary

• Re-apply the gcsfs 2026.6.0 bump from Build: Bump gcsfs from 2026.5.0 to 2026.6.0 #3560, which was reverted in Revert "Build: Bump gcsfs from 2026.5.0 to 2026.6.0" #3580 after integration-test-gcs became very slow.
• Keep fake-GCS sanity tests fast by disabling gcsfs HNS/Zonal bucket detection for test-gcs.

Context

• Build: Bump gcsfs from 2026.5.0 to 2026.6.0 #3560 bumped gcsfs from 2026.5.0 to 2026.6.0, updating google-cloud-storage to 3.12.0.
• Revert "Build: Bump gcsfs from 2026.5.0 to 2026.6.0" #3580 reverted that bump because integration-test-gcs took about 54 minutes instead of about 1 minute.
• This PR keeps the dependency bump and adds the documented gcsfs workaround for fake-gcs-server.

CI Timing

• Current PR Build: Bump gcsfs to 2026.6.0 #3582 integration-test-gcs: 1m19s total, 39s in the GCS test step.
• Previous Build: Bump gcsfs from 2026.5.0 to 2026.6.0 #3560 integration-test-gcs: 54m45s total, 54m00s in the GCS test step.

Testing

• env UV_CACHE_DIR=.cache/uv uv lock --check
• make -n test-gcs
• env UV_CACHE_DIR=.cache/uv uv sync --all-extras --locked
• Verified GCSFS_EXPERIMENTAL_ZB_HNS_SUPPORT=false imports gcsfs 2026.6.0 with gcsfs.core.GCSFileSystem