Support conditional sendfile based on transport and platform

[kjnilsson]

Add use_sendfile flag to read state to conditionally use file:sendfile() for TCP on non-macOS systems, while falling back to pread+send for SSL and macOS. Update tests to be platform-aware using assert_sendfile_pread.

[Copilot]

Pull request overview

Adds transport/platform-aware selection of file:sendfile/5 vs file:pread/3 + socket send when streaming chunks, and updates tests to tolerate macOS behavior differences.

Changes:

• Extend the reader state with a use_sendfile flag derived from transport and OS type.
• Route send_file/3 through a new sendfile/6 implementation that can fall back to pread + send.
• Make sendfile-related tests macOS-aware via assert_sendfile_pread/3.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/osiris_log.erl	Introduces use_sendfile in read state and threads it into send_file/3 to disable sendfile on SSL/macOS.
test/osiris_log_SUITE.erl	Updates sendfile expectations to account for macOS using pread instead of sendfile.

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[michaelklishin]

I confirm that this change eliminates a macOS-specific osiris_replica:parse_chunk/3 function_clause when a replica stops (e.g. in RabbitMQ tests).

The root cause comes down to a write(2) followed by sendfile(2) not guaranteed data delivery order even in the case of TCP sockets, which trips up the above function. Or at least that's our best current understanding.