Serialize Linux URLSession request paths to mitigate _MultiHandle race

[mattt]

Resolves #127
Alternative to / complement of #128

[Copilot]

Pull request overview

This PR addresses a critical use-after-free crash on Linux caused by a thread-safety bug in swift-corelibs-foundation's URLSession._MultiHandle. The fix introduces a serialization gate (LinuxURLSessionRequestGate) that ensures only one URLSession request executes at a time on Linux, mitigating concurrent access to the problematic _MultiHandle singleton.

Changes:

• Adds LinuxURLSessionRequestGate actor to serialize URLSession operations on Linux
• Wraps all data(for:) and streaming calls with the gate's lock on Linux platforms
• Maintains separate code paths for Linux and non-Linux platforms using conditional compilation

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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