Function validateTargetConfiguration

Validates a gateway target:

• targetConfiguration sets exactly one target type;
• that target type is one MDAA currently supports (lambda) — others are rejected as not yet supported so the error is actionable rather than a downstream deploy failure;
• the Lambda tool source is well-formed (a lambdaArn and a toolSchema with exactly one of inlinePayload (non-empty) or s3).

Returns the validated Lambda tool source so callers consume the narrowed value