Fix query parameter corruption in base_url with trailing slash enforcement

When a base_url contained query parameters, _enforce_trailing_slash() was
appending a slash to the entire raw_path (including the query string), which
corrupted the query parameter values.

For example:
- Before: base_url="https://api.com?data=1" became "https://api.com?data=1/"
  (query param corrupted to "data=1/" instead of "data=1")
- After: base_url="https://api.com?data=1" becomes "https://api.com/?data=1"
  (query param correctly preserved as "data=1")

The fix splits the raw_path into path and query components, adds the trailing
slash only to the path portion, then recombines them.

Fixes #3614

httpx/_client.py

@@ -232,9 +232,19 @@ class BaseClient:
         return self._trust_env
 
     def _enforce_trailing_slash(self, url: URL) -> URL:
-        if url.raw_path.endswith(b"/"):
-            return url
-        return url.copy_with(raw_path=url.raw_path + b"/")
+        # Split raw_path into path and query to avoid corrupting query parameters
+        raw_path = url.raw_path
+        if b"?" in raw_path:
+            # URL has query parameters - only check/add slash to path portion
+            path, query = raw_path.split(b"?", 1)
+            if path.endswith(b"/"):
+                return url
+            return url.copy_with(raw_path=path + b"/?" + query)
+        else:
+            # No query parameters - check/add slash to entire raw_path
+            if raw_path.endswith(b"/"):
+                return url
+            return url.copy_with(raw_path=raw_path + b"/")
 
     def _get_proxy_map(
         self, proxy: ProxyTypes | None, allow_env_proxies: bool

tests/client/test_client.py

@@ -460,3 +460,36 @@ def test_client_decode_text_using_explicit_encoding():
         assert response.reason_phrase == "OK"
         assert response.encoding == "ISO-8859-1"
         assert response.text == text
+
+
+def test_base_url_with_query_params():
+    """
+    Test that base_url with query parameters doesn't corrupt the query values.
+
+    Regression test for issue #3614.
+    """
+    client = httpx.Client(base_url="https://example.com/api?data=1")
+
+    # Query parameter should not be corrupted with trailing slash
+    assert client.base_url.query == b"data=1"
+    assert str(client.base_url) == "https://example.com/api/?data=1"
+
+
+def test_base_url_with_trailing_slash_and_query():
+    """
+    Test that base_url with existing trailing slash and query params works correctly.
+    """
+    client = httpx.Client(base_url="https://example.com/api/?key=value")
+
+    assert client.base_url.query == b"key=value"
+    assert str(client.base_url) == "https://example.com/api/?key=value"
+
+
+def test_base_url_with_multiple_query_params():
+    """
+    Test that base_url with multiple query parameters works correctly.
+    """
+    client = httpx.Client(base_url="https://example.com/api?a=1&b=2&c=3")
+
+    assert client.base_url.query == b"a=1&b=2&c=3"
+    assert str(client.base_url) == "https://example.com/api/?a=1&b=2&c=3"