Record the real GCS run and guard the console-folder path in its test

[FabianIMV]

Follows up on the live run reported in #214.

• Replaces the "construction tests only" note with what was run: 2026-10-05, us-central1, flat namespace, uniform access, service account with only Storage Object Viewer.
• Names the fixture in the doc comment: docs/a.txt containing hello, plus one folder made in the Cloud Console.
• Checks the content of a.txt, as the S3 test does.
• Asserts that the raw listing has a zero-byte placeholder and that fetch returns none, so a green run can't come from a bucket without one.

Ran cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace, and it_reads_from_gcs against the real bucket.

Closes #214

[WaylandYang]

Exactly what was agreed on #214, and nothing extra. Two things I checked:

• Counting placeholders on the raw listing before calling fetch is the right order: fetch drops them by size, so its output alone could not tell a filtered bucket from an empty one.
• The fixture is now written where the next person will look, so a first run no longer panics on a bare a.txt.

CI is green; the live test stays skipped there, as intended. Merging, and I will close #214 by hand since the PR targets dev.

Thank you for the run and for following it through to a PR.