test: skip symlink tests instead of erroring when the platform can't create symlinks

[rajarshidattapy]

Fixes #2642.

The bug

Creating a symlink on Windows needs an elevated process or Developer Mode. Fifteen tests call
Path.symlink_to() / os.symlink() unguarded, so on a default Windows developer machine they
raise OSError: [WinError 1314] A required privilege is not held by the client and are
reported as failures.

At 15 of 42 failures on a clean checkout, this was the single largest block of noise in the
suite — enough that a Windows run isn't usable as a signal.

Fix

The capability-probing fixture pair from the issue, in tests/conftest.py: a session-scoped
_can_symlink probe plus a requires_symlinks gate, taken as a parameter by the 15 tests.

Probed rather than inferred from sys.platform, as the issue specifies — an elevated shell or
Developer Mode can create symlinks on Windows, and those runs must keep the coverage.

Results

before:  42 failed, 4120 passed, 16 skipped
after:   27 failed, 4252 passed, 31 skipped

The failure-set diff is exact: the 15 converted are precisely the reported ones, and zero new
failures.

Verifying the gate opens as well as closes

A guard that always skips would "fix" the count while silently deleting the coverage — the more
expensive failure of the two. Exercising the fixture functions directly:

probe on this machine -> False
  _can_symlink=True  -> runs the test
  _can_symlink=False -> SKIP (symlink creation unavailable on this machine ...)
probe leaked temp entries: none

Scope

There are 17 unguarded symlink call sites, not 15. The extra two are already skipif-decorated
for unrelated reasons and were left alone:

test	existing skip reason
test_build_merge_hyperedges_and_prune.py::test_prune_matches_across_symlinked_root	"POSIX symlink semantics"
test_watch.py::test_rebuild_code_incremental_rename_preserves_symlink_source_path	"symlink setup differs on Windows"

Three further tests (test_cache.py, two in test_detect.py) already use the repo's existing
try/except + pytest.skip idiom. Left as-is rather than churn working code, though
requires_symlinks is the better pattern for anyone touching them later.

Design note

requires_symlinks is a parameter rather than a try/except wrapper around each call on purpose:
the guard stays visible in the signature, and an OSError raised by the code under test
stays a real failure instead of being swallowed into a skip. The probe creates one file symlink
— Windows gates file and directory symlinks behind the same SeCreateSymbolicLinkPrivilege, so
a separate directory probe would add nothing.

Test infrastructure only; no product code touched. #2478 remains the tracker for the genuine
junction/symlink scan-root defect.

[graphify-labs]

Graphify reviewed this change.

Looks safe to merge — no coupling regressions and no blocking issues, checked against the code graph (not a self-assessment).

---

Graphify review — findings

This PR adds two new pytest fixtures to tests/conftest.py: a session-scoped _can_symlink that probes whether the machine can actually create symlinks, and a requires_symlinks fixture that skips a test when it can't. It then adds requires_symlinks as a parameter to the various existing tests across test_atomic_writes.py, test_detect.py, test_extract.py, and test_image_vision.py that create symlinks. The stated intent is to make symlink-dependent tests skip cleanly (e.g., on non-elevated Windows) rather than fail. The change is confined to test files and fixtures; no non-test code is touched.

No blocking issues surfaced.

Analysis details — impact, health, verification

Impact & health

Graphify review

Impact — 715 functions depend on the 715 functions this change touches.

Health — grade A; no new coupling hotspots.

Verification — 715 functions in the blast radius were not formally verified this run (proofs are advisory here).

Gate & verification

graphify gate

PASS — objectively clean (no health regressions, tests not run — proofs not run this pass (advisory)). Grounded, not self-assessed.

Advisory (not blocking):

• verification_scope: 715 function(s) in the blast radius were not formally verified this run