Darwin: fix sockaddr_un overflow in makeAddressUnix for long paths

[ianegordon]

Summary

• Socket.makeAddressUnix(path:) on Darwin clamped the path to 104 bytes but bounded strncpy by sun_len (up to 106), writing past the end of sun_path — which Darwin <sys/un.h> declares as char sun_path[104]. Paths ≥ 102 bytes overflowed the struct.
• The path is now truncated to 103 bytes and sun_path is always NUL-terminated, so String(cString:) and unlink() read back a valid C string.
• Fixes the same off-by-one in the existing maximumPathLengthForUnixDomainSocket test, which computed the capacity from struct arithmetic that omitted sun_len and copied 105 bytes into the 104-byte field (caught by AddressSanitizer).

Test plan

• Added round-trip tests for a max-length (103-byte) path and an overlong path that must truncate without overflow.
• swift test --filter SocketAddressTests — 20/20 pass.
• swift test --sanitize address --filter SocketAddressTests — clean, no ASan reports.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.00%. Comparing base (4e246d3) to head (bb98152).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
+ Coverage   92.88%   93.00%   +0.11%     
==========================================
  Files          71       71              
  Lines        3727     3730       +3     
==========================================
+ Hits         3462     3469       +7     
+ Misses        265      261       -4     

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