Validates a tool call's input before the tool runs.

The dispatcher checks the model's input here and passes the result to Normandy.Tool.run/2 as context.input; it never goes into the tool struct.

• A tool that declared input: Schema (use Normandy.Tool) gets a %Schema{}. Every key (string or atom) must name a field of the schema; the values are checked by Normandy.Schema.Validator (types, required fields, enums, ranges, formats; nothing is coerced, so "5" for an integer field is an error); missing optional fields take their defaults.
• Any other tool gets a string-keyed map checked against its Normandy.Tool.input_schema/1 map (string or atom keys): every required key present, each value's basic JSON type (string, integer, number, boolean, object, array, null) where its property declares one, and unknown keys as follows. A schema that declares properties is closed: no key outside them unless additionalProperties is true or a schema (an absent additionalProperties counts as false here, unlike JSON Schema's default). A schema with no properties key declares no keys, so it limits none: any key is accepted (an MCP server's loose {"type": "object"} keeps working). Nested schemas, enums and formats are the tool's to check. A type name outside the seven above (a typo such as "intger") is not checked: any value passes it. An input_schema/1 that returns something other than a map, or a map whose properties is not a map or whose required is not a list of key names, is an error (constraint: :schema).

An unknown key is an error for a declared input schema and for a map that declares properties, and keys are matched against declared names, so model input never creates an atom. A key that is neither a string nor an atom is always an unknown key, named by its type ("a tuple key"). Errors are %{path: [key], message: String.t(), constraint: atom()}; message/1 joins them into one line for the model.