Return buffered XML write errors

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I run Live Tennis API.

Write and WriteWithOptions return nil when a short XML write fails during the final buffer flush. Return that error and preserve earlier serialization errors.

The tests cover document and element output, indentation, partial writes and successful writes. All 56 cases pass. On the original source, 21 cases for short output fail.

go test . -v -cover passes on Go 1.23 and 1.24, with 89.1% coverage. go vet ./... and go build ./... pass.