fix(gstr): preserve repeated query parameters in Parse (fix #4751)

[waterWang]

fix(gstr): preserve repeated query parameters in Parse (fix #4751)

What

Fix gstr.Parse to preserve repeated query parameters when a plain key appears multiple times. Previously, ?name=a&name=b would only keep the last value ("b"), now it correctly produces ["a", "b"].

Why

When a GoFrame request handler receives a URL like ?names=a&names=b and the target struct field is []string, r.Parse(&req) should bind both values. The gstr.Parse function's build method was overwriting the existing value instead of collecting all values into a slice.

How

Changed the build function's length == 1 branch to check if the key already exists in the result map. If it does, both the existing and new values are converted to a []any slice and appended. This is consistent with the existing [] notation behavior (v[]=a&v[]=b → [a b]), but without requiring explicit slice notation.

Documentation updates

The Parse function's doc comment is updated to reflect the new behavior:

• v=m&v=n -> map[v:[m n]] (was map[v:n])

Testing

Before:

?names=a&names=b → Parse returns {"names": "b"} → bind to []string → ["b"]

After:

?names=a&names=b → Parse returns {"names": ["a", "b"]} → bind to []string → ["a", "b"]

Fixes #4751