Fix socket fd leaks on AsyncSocket connect/accept failure paths

[ianegordon]

Summary

Two file-descriptor leaks in AsyncSocket:

• connected(to:pool:timeout:) — the freshly created Socket was never closed when AsyncSocket.init threw, connect(to:) failed terminally (e.g. ECONNREFUSED), or the timeout fired. Every failed or timed-out client connect leaked one fd. Since withThrowingTimeout awaits the body task's result after cancelling it, a catch inside the body is guaranteed to run on the timeout path too.
• accept() — if AsyncSocket.init threw after socket.accept() returned a valid descriptor, the accepted fd was orphaned.

Both sites now close the underlying socket before rethrowing the original error (try? close() so the original error is preserved).

Testing

• New connected_ThrowsError_WhenConnectFails connects to a nonexistent unix-socket path — deterministic (synchronous failure, no network timing) — and code coverage confirms it executes the new close-on-failure path.
• The accept() catch path isn't reachable in unit form (would require forcing fcntl F_SETFL to fail on a valid fd); it mirrors the tested connect path.
• Full suite passes: 450 tests in 51 suites.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.85%. Comparing base (4e246d3) to head (6508863).

Files with missing lines	Patch %	Lines
FlyingSocks/Sources/AsyncSocket.swift	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
- Coverage   92.88%   92.85%   -0.04%     
==========================================
  Files          71       71              
  Lines        3727     3735       +8     
==========================================
+ Hits         3462     3468       +6     
- Misses        265      267       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.