Validate remote read byte counts

[fallintoplace]

Summary

• validate successful easy-curl transfers against the callback byte count
• reset callback byte accounting before each easy-curl retry attempt
• reset the existing device bounce state before retrying device reads
• fail easy and multi-poll remote reads when the callback receives fewer bytes than requested

Why

A remote read could return success after libcurl completed even if the write callback received fewer bytes than requested. The easy retry path also reused the same callback offset between attempts, so a partial timed-out attempt could make the next attempt append at the wrong offset.

This keeps the existing host and device callback paths, restarts per-attempt byte state on retries, and rejects successful transfers that did not deliver the requested byte count.

Validation

• pre-commit run --files cpp/include/kvikio/detail/remote_callback.hpp cpp/src/detail/remote_callback.cpp cpp/include/kvikio/shim/libcurl.hpp cpp/src/shim/libcurl.cpp cpp/src/remote_handle.cpp cpp/src/detail/multi_poll_reactor.cpp python/kvikio/tests/test_http_io.py
• python3 -m py_compile python/kvikio/tests/test_http_io.py
• git diff --check

Not run locally: the KvikIO Python/C++ test suite, because this checkout does not have an importable kvikio package and CUDA tooling/GPU access are unavailable (nvcc and nvidia-smi are not present).

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[madsbk]

/ok to test 164a360

[wence-]

This change seems to needlessly introduce another level of intermediate bounce buffer. Why is it not possible to just use the existing setup and check the relevant error condition as part of curl.perform?

[fallintoplace]

Thank you for your attention, I have amended.