[Copilot]

validate.Spec and validate.AgainstSchema previously accepted unbounded allOf/anyOf/oneOf/not nesting, so small untrusted documents could trigger disproportionate CPU and memory usage. This change introduces a fail-fast composition-depth guard with deterministic error locations.

• Configurable depth limit (safe default)

  • Adds WithMaxSchemaCompositionDepth(limit int) to schema validation options.
  • Sets a default maximum depth of 256; non-positive values fall back to this default.

• Shared depth-check implementation

  • Adds an iterative composition walker (schema_composition_depth.go) that traverses only allOf, anyOf, oneOf, and not.
  • Tracks depth explicitly and reports the first over-limit path deterministically.

• Enforcement at both validation entry points

  • Applies the guard in schema validation (AgainstSchema / NewSchemaValidator) before deep validator construction.
  • Applies the guard in spec validation (SpecValidator.Validate) as an early pass across schema-bearing locations (definitions, parameters, responses, and path/operation schema slots).

• Error behavior

  • Adds a dedicated validation message: schema composition nesting exceeds limit.
  • Surfaces this as standard validation errors, preserving normal error reporting semantics.

• Regression coverage

  • Adds tests for:
    • AgainstSchema: over-limit fails fast, higher limit succeeds.
    • Spec: deeply nested definition composition fails fast, higher limit succeeds.
    • Option behavior: default and round-trip handling for WithMaxSchemaCompositionDepth.

err := validate.AgainstSchema(schema, data, strfmt.Default,
	validate.WithMaxSchemaCompositionDepth(64),
)
// err contains: "<path> schema composition nesting exceeds limit of 64"