Minimal JSON Schema validator for MCP tool inputs.

Implements the subset of JSON Schema that real MCP tools use, in pure Erlang with no extra dependencies. Covers:

• type: object, array, string, number, integer, boolean, null, or a list of those for unions.
• properties + required for objects.
• additionalProperties: false rejection.
• items for arrays (single schema applied to every item).
• enum for string/number/integer/boolean.
• oneOf / anyOf / allOf (allOf = each must validate).
• minimum/maximum/exclusiveMinimum/exclusiveMaximum for numbers and integers.
• minLength/maxLength/pattern for strings.
• minItems/maxItems/uniqueItems for arrays.

Returns ok or {error, [Error]} where each error is a tuple {Path, Reason} (Path is a list of keys / indices from the root, Reason an atom or {atom, Detail}).

Anything outside the supported subset is silently accepted; this is a permissive validator by design — its job is to catch obvious bugs before sending an LLM-generated arg map to a remote tool, not to be a JSON Schema reference implementation.