Merge params with existing URL query parameters instead of replacing them

When a URL already contains query parameters and the `params` argument is
also provided, the existing query parameters are now merged with the new
ones instead of being silently dropped. This aligns with the behavior of
the `requests` library and matches user expectations.

Fixes #3621

httpx/_models.py

@@ -396,7 +396,7 @@ class Request:
         extensions: RequestExtensions | None = None,
     ) -> None:
         self.method = method.upper()
-        self.url = URL(url) if params is None else URL(url, params=params)
+        self.url = URL(url) if params is None else URL(url).copy_merge_params(params)
         self.headers = Headers(headers)
         self.extensions = {} if extensions is None else dict(extensions)
 

tests/models/test_requests.py

@@ -232,10 +232,18 @@ def test_request_params():
     request = httpx.Request("GET", "http://example.com", params={})
     assert str(request.url) == "http://example.com"
 
+    # Params are merged with existing URL query parameters, not replaced.
     request = httpx.Request(
         "GET", "http://example.com?c=3", params={"a": "1", "b": "2"}
     )
-    assert str(request.url) == "http://example.com?a=1&b=2"
+    assert str(request.url) == "http://example.com?c=3&a=1&b=2"
 
+    # Empty params preserves existing URL query parameters.
     request = httpx.Request("GET", "http://example.com?a=1", params={})
-    assert str(request.url) == "http://example.com"
+    assert str(request.url) == "http://example.com?a=1"
+
+    # Params with overlapping keys override existing URL query parameters.
+    request = httpx.Request(
+        "GET", "http://example.com?a=1&b=2", params={"b": "3", "c": "4"}
+    )
+    assert str(request.url) == "http://example.com?a=1&b=3&c=4"