linux: fix readlinkat signed error handling and add regression test

[jlinenkohl]

Summary

• Harden SYS_readlinkat guest copy path to avoid wrapped-length copy on host readlinkat() failure
• Keep syscall return contract behavior unchanged
• Add a focused unit regression test for the failing readlinkat path

Why

This bug is independently valuable to fix and was discovered while working relocation test lanes.

Validation

• cd tests && bash ./run_unit_tests.sh -R test_basic

Notes

This is intentionally standalone and separated from relocation/test-gating work.

[fwsGonzo]

The new std::vector on the heap is <= PATH_MAX in size, and is just more costly than a fixed-size PATH_MAX buffer. I think we will keep the fixed-size buffer and read the link contents into that.

[jlinenkohl]

Agreed. I have made that adjustment.

[fwsGonzo]

Thanks!