:sig
StringMap parse_query(String q)

:params
q : string containing URL parameters
return value : a StringMap containing the parameters

:see
>uri
encode_query
request_context_params

:content
Decodes a query-string fragment such as `a=b&c=d` into a `StringMap`.

Parsing rules:

- pairs are separated on `&`
- each pair is split on the first raw `=` only
- both key and value are URL-decoded
- keyless flags such as `preview` are present with an empty string value
- empty pairs, including a trailing `&`, are ignored
- repeated keys use the last value seen

Examples:

```cpp
StringMap q = parse_query("alpha=1&token=a%3Db&preview&empty=");
// q["alpha"] == "1"
// q["token"] == "a=b"
// q["preview"] == ""
// q["empty"] == ""
```

This is useful when you need to work with URL parameter data outside the normal request parsing flow.

Related:

- PHP: `parse_str()`
- JavaScript / Node.js: `URLSearchParams` or Node `querystring.parse()`