Fix D205 docstring lint on test_raises_on_dtype_contradiction

The new test docstring added in 3b0fa13 tripped the repo's enforced
pydocstyle gate (ruff "D" select, google convention): D205 (blank line
required after the summary line), D209 (closing quotes on their own
line), and the resulting ruff format diff, failing the PR Lint check.

Rewrite the docstring as a single-line summary followed by a blank line
and a wrapped description. Lint-only change; no test logic or assertions
are touched (22 dtype/fold-concat tests still pass).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: titaiwang <titaiwang@microsoft.com>

Author: titaiwangms

src/mobius/_passes/_dtype_utils_test.py

@@ -36,8 +36,11 @@ def test_falls_back_to_const_value_when_declared_missing(self):
         assert initializer_dtype(v) == ir.DataType.FLOAT16
 
     def test_raises_on_dtype_contradiction(self):
-        """A declared type that contradicts the serialized data is corrupt
-        metadata and must fail closed rather than silently pick one dtype."""
+        """Reject values whose declared dtype contradicts the serialized data.
+
+        Such a value is corrupt metadata and must fail closed rather than
+        silently pick one dtype.
+        """
         v = _value(ir.DataType.FLOAT, np.ones((2,), np.float16))
         with pytest.raises(ValueError) as excinfo:
             initializer_dtype(v)