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GH-38821: [C++] Strengthen handling of duplicate slashes in S3, GCS (#40371)

### Rationale for this change

Giving a path such as "bucket//key" to some S3 and GCS filesystem APIs could silently succeed or crash.

Make sure those paths instead return an error, as other instances of duplicate slashes already do.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes. Some paths that were accepted by some filesystems can now return an error.

* GitHub Issue: #38821

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>