Fix LinuxHardwareI2C fd lifecycle

[l5yth]

• Default i2c_file to -1; was 0 (stdin), so pre-begin() Wire ops blocked on stdin instead of failing fast.
• Only set hasBegun when open() succeeds, so a failed begin() can retry.
• Reset i2c_file to -1 in end() to avoid using a stale (or reused) fd.
• Fix Wire (LinuxHardwareI2C) defaults its fd to 0 (stdin), I2C ops before begin() block on stdin or run against the wrong fd #71

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[Copilot]

Pull request overview

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Fixes the Linux I2C file-descriptor lifecycle so pre-begin() operations don’t accidentally act on fd 0, and failed begin() calls can be retried safely.

Changes:

• Initialize i2c_file to -1 (unopened) rather than 0.
• Set hasBegun only when open() succeeds.
• Reset i2c_file to -1 in end() after closing.

Reviewed changes

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

File	Description
cores/portduino/linux/LinuxHardwareI2C.h	Default i2c_file to -1 to avoid accidental use of fd 0 prior to open().
cores/portduino/linux/LinuxHardwareI2C.cpp	Guard hasBegun on successful open() and clear i2c_file on end().

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