fix(cpp): flush pending data on writer close

[ColinLeeo]

Summary

• Flush pending buffered data before finalizing a C++ TsFile writer.
• Propagate flush errors without writing the index/footer.
• Remove redundant flush calls from C ABI close wrappers and add regression coverage.

Testing

• cmake --build cpp/target/build --target TsFile_Test -j2
• Relevant writer/C-wrapper tests: 55 passed; final focused rerun: 12 passed.

[Copilot]

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Copilot review overview

Review effort: Lite
Findings: 1 · 3

Open (4)

• This closes the result set twice (explicit close() and then destroy_query_data_set(...) which… · New
• This doc now guarantees \"releases resources\" on close, but TsFileWriter::close() can return… · New
• On flush() failure this returns early without calling io_writer_->end_file() (or any… · New
• Casting the ResultSet* returned from reader.query(...) to a concrete derived type via… · New

What changed in this PR

Flushes buffered data as part of writer close to avoid missing trailing writes, and simplifies C ABI close wrappers by relying on the writer’s close semantics.

Changes:

• Call flush() inside TsFileWriter::close() and propagate flush failures before finalization.
• Remove redundant explicit flush() calls from C wrapper close functions.
• Add/extend regression tests verifying close persists data written after the last flush (writer + table view).

File	Description
cpp/​test/​writer/​tsfile_writer_test.cc	Adds a regression test ensuring close() flushes data written after the last flush().
cpp/​test/​writer/​table_view/​tsfile_writer_table_test.cc	Updates table writer test to verify data is readable after close() (without explicit flush()).
cpp/​src/​writer/​tsfile_writer.h	Updates close() documentation to reflect the new flush-and-finalize behavior.
cpp/​src/​writer/​tsfile_writer.cc	Implements close() to flush() before calling end_file().
cpp/​src/​writer/​tsfile_table_writer.h	Updates close() documentation to mention flushing/finalization.
cpp/​src/​cwrapper/​tsfile_cwrapper.h	Updates C API docstring to mention flush/finalize behavior on close.
cpp/​src/​cwrapper/​tsfile_cwrapper.cc	Removes explicit flush() from C ABI close wrappers and relies on close().

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