Report a missing hard link source before an existing destination - #1357

Describe the changes

os.link(src, dst) with a missing src and an existing dst raises EEXIST under pyfakefs, but the real call raises ENOENT:

open("dst", "w").close()
os.link("missing", "dst")
# real (macOS and Linux): FileNotFoundError: [Errno 2]
# pyfakefs:               FileExistsError:   [Errno 17]

On Posix, link() resolves the source before it creates the destination entry. On Linux, do_linkat looks up oldname before filename_create(newname). create_link checked the destination first.

This PR moves the source checks into a small _resolve_link_source helper:

• Under Posix, the helper runs before the destination check.
• Under Windows, it runs where the checks were before, so Windows behaviour is unchanged.
• The trailing-separator ENOTDIR check on the source stays with the lookup, so it keeps its order.

I found this with a differential fuzz that runs random sequences of os/shutil calls on the real file system and on pyfakefs, then compares the results.

Tasks

• Unit tests added that reproduce the issue or prove feature is working
  • Added test_link_bogus_to_existing_file_posix.
  • It fails without the fix and passes with it.
  • It also passes with TEST_REAL_FS=1 on macOS.
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
  • ruff check and ruff format --check are clean locally.
• Unit tests passing
  • pyfakefs/tests: 1443 passed, 1046 skipped.
  • The link tests also pass with TEST_REAL_FS=1.

AI-assisted (Claude): I used it to help read the code and draft the test. I reviewed and ran everything myself.