SOR - Make input validation more robust

```
export function isValidSwapRequest(tokenIn: string, tokenOut: string, swapAmount: string, chain: Chain): boolean {
    if (parseFloat(swapAmount) <= 0) {
```

I think this might throw if some random string is provided or a number in scientific notation. Should check this here as well

_Originally posted by @franzns in https://github.com/balancer/backend/pull/1808#discussion_r2020892028_
            