_repair_object_shape() treated every dict carrying `required` as an object
declaration. A constraint fragment — {"required": ["chain"]} inside an
allOf/oneOf/anyOf/if branch, with no properties and no type — is not one.
Repairing it synthesised `properties: {}` and then pruned every name out of
`required`, so sibling branches collapsed into identical always-true schemas
and the enclosing oneOf had two matches: the tool appeared in the catalog and
every call failed validation.
Only the parameters-schema root still receives the dangling-`required` repair,
which is the single node handed to providers as the argument object.
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