feat(c): expose TableWrite close for explicit cleanup - #1306

[P2] Document the I/O and failure semantics of close

#1302 asks for cancellation without flushing unprepared output. This wrapper calls TableWrite::close(), whose append path finishes the active Parquet writer before attempting to delete its file. Cleanup errors are discarded by core and the C function still returns success.

I reproduced this via the C write and close APIs with an instrumented storage provider. The unprepared object was absent before close. Close wrote it and then attempted deletion. With an injected deletion denial, close returned null and the object remained. With successful deletion, it disappeared. In both runs, the previously prepared message still committed and only its row was visible.

Please describe this API as best-effort cleanup that can finish and upload buffered output and that does not report storage cleanup errors. If cancellation without flushing is required for #1302, it needs a separate core abort path.

[P2] Document that close may flush buffered output

Please clarify that close() may finish and upload buffered output before attempting deletion. The append path calls writer.close().await during abort, so callers cannot rely on this API for cancellation without flushing. The current text explains cleanup failures but omits this I/O behavior. A documentation clarification is sufficient for this PR.