fix(uri): ? operator now appends to existing query string (#25831)

## Summary

Fixes #19782.

The `?` operator in `std/uri` was silently overwriting any query string
already present in the URI. This PR makes it append instead — which
matches the docstring ("Concatenates the query parameters") and the
natural expectation when chaining operations.

**Before:**
```nim
let u = parseUri("https://example.com/foo?existing=1") ? {"bar": "qux"}
echo $u  # https://example.com/foo?bar=qux  (existing=1 lost)
```

**After:**
```nim
let u = parseUri("https://example.com/foo?existing=1") ? {"bar": "qux"}
echo $u  # https://example.com/foo?existing=1&bar=qux
```

## Changes

- `lib/pure/uri.nim`: fix `?` to append with `&` when a query string
already exists; add example to `runnableExamples`
- `tests/stdlib/turi.nim`: two new test cases (append to existing query,
empty params preserve existing)
- `changelog.md`: entry under Standard library changes

## Notes

I work with Claude as a co-processor. I'm 56, came to programming late,
and this is genuinely how I learn and contribute. I understand what I'm
submitting, but I didn't write it alone. If your project prefers
human-only contributions, just say so and I'll close without friction.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: n0madgang <14005836+n0madgang@users.noreply.github.com>

tests/stdlib/turi.nim

@@ -289,6 +289,15 @@ template main() =
       var foo = parseUri("http://example.com") / "foo" ? {"do": "do", "bar": ""}
       var foo1 = parseUri("http://example.com/foo?do=do&bar")
       doAssert foo == foo1
+    block: # issue #19782: appends to existing query string
+      var foo = parseUri("http://example.com/foo?existing=1") ? {"bar": "qux"}
+      doAssert $foo == "http://example.com/foo?existing=1&bar=qux"
+    block: # issue #19782: empty params list preserves existing query
+      var foo = parseUri("http://example.com/foo?existing=1") ? {:}
+      doAssert $foo == "http://example.com/foo?existing=1"
+    block: # issue #19782: empty params on uri without query is a no-op
+      var foo = parseUri("http://example.com/foo") ? {:}
+      doAssert $foo == "http://example.com/foo"
 
   block: # getDataUri, dataUriBase64
     doAssert getDataUri("", "text/plain") == "data:text/plain;charset=utf-8;base64,"