Reject non-finite Float values

[ydah]

GraphQL Float values must be finite, but GraphQL::Types::Float currently accepts any Numeric value after calling to_f.
This allows non-finite values through both input and result coercion:

Schema.execute("{ echo(value: 1e400) }")
# => Infinity is accepted as input

result = Schema.execute("{ nan }")
JSON.generate(result.to_h)
# => JSON::GeneratorError: NaN not allowed in JSON

Variables containing Float::NAN or infinity are also accepted. For result values, the failure occurs later during HTTP response serialization instead of being reported by GraphQL-Ruby.

This PR adds finite-value checks to Float coercion, following the existing Int error-handling pattern. Invalid inputs are routed through Schema#type_error using FloatDecodingError, while invalid result values raise FloatEncodingError with field and response-path context.

Variable validation errors and serialized GraphQL values also represent non-finite floats as strings, keeping error responses JSON-safe.

[rmosolgo]

Looks good to me. This might result in errors being raised where previously these values were being allowed through, but this is correct according to the spec:

Non-finite floating-point internal values (NaN and Infinity) cannot be coerced to Float and must raise a field error.

https://spec.graphql.org/September2025/#sel-GAHXTFHCAACIBx4Q