Validation errors are too verbose

While investigating certain validation errors for the Go SDK, I noticed that they're rather verbose, which is undesirable for both human and LLM consumption. For example:

> "validating\n\t\u0026map[]\nagainst\n\t {\"type\":\"object\",\"required\":[\"In\"],\"properties\":{\"In\":{\"type\":\"string\",\"description\":\"the input\"}},\"additionalProperties\":false}:\n validating root: required: missing properties: [\"In\"]"

This is just saying that the input is missing required property "In", but it's obscured by printing the input and the schema.

We should simplify these errors, or provide an error API to access the inner error.

CC @jba @samthanawalla @mazas-google