Deterministically cover the scan-time DependencyError branch

tests/test_io/test_io_core.py

@@ -18,6 +18,7 @@
 import dascore as dc
 from dascore.config import config_context
 from dascore.exceptions import (
+    DependencyError,
     InvalidFiberIOError,
     MissingOptionalDependencyError,
     MissingPatchError,
@@ -199,6 +200,25 @@ def get_format(self, resource: Path, **kwargs) -> tuple[str, str] | Literal[Fals
         return False
 
 
+class _DependencyErrorFormatter(FiberIO):
+    """A formatter whose scan path hits a dependency/compatibility problem."""
+
+    name = "_dependency_error_formatter"
+    version = "1"
+
+    def scan(self, resource: Path, **kwargs):
+        """Raise a stable dependency error for scan coverage tests."""
+        msg = "simulated stack incompatibility while scanning"
+        raise DependencyError(msg)
+
+    def get_format(self, resource: Path, **kwargs) -> tuple[str, str] | Literal[False]:
+        """Only accept the explicit dependency-error test resource."""
+        path = Path(resource)
+        if path.suffix == ".dep" and path.name == "dependency_error.dep":
+            return self.name, self.version
+        return False
+
+
 class TestGetExactCoord:
     """Tests for constructing exact coordinates during scans."""
 
@@ -957,6 +977,26 @@ def test_scan_missing_optional_dependency_warns_with_other_outputs(self, tmp_pat
         assert len(out) == 1
         assert out[0].source_format == _ReadOnlySummaryFormatter.name
 
+    def test_scan_dependency_error_warns_and_skips(self, tmp_path):
+        """
+        A scan-time dependency/compatibility problem warns and keeps scanning.
+
+        Scan is best-effort across many resources, so such problems must
+        surface as warnings on the affected file rather than aborting the
+        whole scan. (The DASVader legacy file used to exercise this branch,
+        but whether it does depends on the installed HDF5 stack.)
+        """
+        dep_path = tmp_path / "dependency_error.dep"
+        dep_path.write_text("placeholder")
+        readable_path = tmp_path / "fallback_scan.h5"
+        readable_path.write_text("placeholder")
+
+        with pytest.warns(UserWarning, match="simulated stack incompatibility"):
+            out = dc.scan([dep_path, readable_path])
+
+        assert len(out) == 1
+        assert out[0].source_format == _ReadOnlySummaryFormatter.name
+
     def test_local_upath_file_interfaces(self, terra15_v6_path):
         """Ensure core file IO accepts local UPath inputs."""
         path = UPath(terra15_v6_path)