* fix(gcssink): prevent empty object finalization on write failure
The GCS writer was created unconditionally with defer wc.Close(),
which finalizes the upload even when content decryption or copy
fails. This silently overwrites valid objects with empty data.
Remove the unconditional defer, explicitly close on success to
propagate errors, and delete the object on write failure.
* fix(gcssink): use context cancellation instead of obj.Delete on failure
obj.Delete() after a failed write would delete the existing object at
that key, causing data loss on updates. Use a cancelable context
instead — cancelling before Close() aborts the GCS upload without
touching any pre-existing object.
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