test(learning-harness): apply the group 1 review suggestions

A mechanism claimed by two families now fails as loudly as a weakness claimed
twice, with a named offender. Two behaviours that were correct but unasserted
are pinned: the obligations tool refuses a path outside the repository, and a
provider that refuses to build degrades to no endpoint instead of raising into
a scan. The TypeScript parse assertion is now exact rather than a quarter-of-
the-slice allowance, which surfaces one real corpus defect: a TSX excerpt
harvested as a truncated destructuring parameter list is not valid TypeScript
on its own, and belongs to the re-harvest rather than to the grammar.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

tests/test_learning_families.py

@@ -88,6 +88,10 @@ def test_relation_answers_same_lateral_fabricated_and_parent_child_when_declared
         (GOOD.replace('cwes = ["CWE-89"]', 'cwes = ["CWE-89", "CWE-89"]'), "CWE-89"),
         (GOOD.replace('description = "x"\n', ""), "description"),
         (GOOD + '\n[[family]]\nid = "path"\ndescription = "y"\ncwes = ["CWE-89"]\nmechanisms = []\nverifier = "canary"\n', "CWE-89"),
+        (
+            GOOD + '\n[[family]]\nid = "path"\ndescription = "y"\ncwes = []\nmechanisms = ["source_reaches_sink"]\nverifier = "canary"\n',
+            "source_reaches_sink",
+        ),
         (GOOD.replace('version = "1"\n', ""), "version"),
         (GOOD + '\n[[family]]\nid = "path"\nparent = "nope"\ndescription = "y"\ncwes = []\nmechanisms = []\nverifier = "none"\n', "nope"),
     ],
@@ -99,6 +103,7 @@ def test_relation_answers_same_lateral_fabricated_and_parent_child_when_declared
         "duplicate_cwe",
         "no_description",
         "cwe_claimed_twice",
+        "mechanism_claimed_twice",
         "no_version",
         "unknown_parent",
     ],