[Bug]: `within` doesn't replace the lefthand side of an expression.

### What happened?

I wanted to mutate a variable by using as.factor but I found that using this approach doesn't replace the content but creates a new column:

``` r
library("teal.code")
var <- "Species"
q <- within(qenv(),
{
library("dplyr")
iris <- iris
iris2 <- mutate(iris, a = as.factor(a))
},
a = as.name(var)
)
setdiff(colnames(q$iris2), colnames(q$iris))
#> [1] "a"
```

^{Created on 2025-10-24 with [reprex v2.1.1](https://reprex.tidyverse.org)}

One workaround is to use the dplyr's walrus `:=` assignment:

``` r
library("teal.code")
var <- "Species"
q <- within(qenv(),
{
library("dplyr")
iris <- iris
iris2 <- mutate(iris, a := as.factor(a))
},
a = as.name(var)
)
setdiff(colnames(q$iris2), colnames(q$iris))
#> character(0)
```

^{Created on 2025-10-24 with [reprex v2.1.1](https://reprex.tidyverse.org)}

but this might confuse readers of the code if this end up on the Show R code on teal.
If not a bug perhaps mentioning this on the documentation would be helpful to users.

Also, I realized that `within` doesn't have the `@aliases within` so users searching with `?within` will not find it (but it is linked from eval_code and qenv).

### sessionInfo()

```R

```

### Relevant log output

```R

```

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