Fix GCSToGCSOperator ignoring replace=False for single-file copy

[9re-pe]

Summary

Respect replace=False in the single-file fallback path of
GCSToGCSOperator._copy_source_without_wildcard. When the source is a
single object (no wildcard), _ignore_existing_files correctly empties
objects if the destination already has the file, but the subsequent
if len(objects) == 0 and prefix: branch then called
_copy_single_object unconditionally, overwriting the destination.

This adds a destination existence check guarded by not self.replace
inside that branch, skipping the copy when the destination already exists.

Alternative considered

I also considered limiting the fallback itself to the case where the
source list was empty before _ignore_existing_files ran. The chosen
approach is smaller and keeps the intent local to where the copy happens;
happy to switch to the alternative if reviewers prefer it.

closes: #66009

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.7)

Generated-by: Claude Code (Opus 4.7) following the guidelines

[boring-cyborg]

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[eladkal]

cc @VladaZakharova for review

[shahar1]

Great work!

[boring-cyborg]

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