fix: prevent URL-encoding of $alt query parameter in REST transport

The `requests` library encodes `$` as `%24` when using the `params` argument,
which causes API errors like:
"Could not find field '%24alt' in the type '...FindNeighborsRequest'"

This change builds the query string manually using `urlencode(safe='$')` to
preserve the `$` character in parameters like `$alt`.

Fixes #2514

Author: taimo3810

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/_shared_macros.j2

@@ -171,16 +171,23 @@ def _get_http_options():
     timeout,
     transcoded_request,
     body=None):
-    
+
     uri = transcoded_request['uri']
     method = transcoded_request['method']
     headers = dict(metadata)
     headers['Content-Type'] = 'application/json'
+    # Build query string manually to avoid URL-encoding special characters like '$'.
+    # The `requests` library encodes '$' as '%24' when using the `params` argument,
+    # which causes API errors for parameters like '$alt'. See:
+    # https://github.com/googleapis/gapic-generator-python/issues/2514
+    _query_params = rest_helpers.flatten_query_params(query_params, strict=True)
+    _request_url = "{host}{uri}".format(host=host, uri=uri)
+    if _query_params:
+        _request_url = "{}?{}".format(_request_url, urlencode(_query_params, safe="$"))  # pragma: NO COVER
     response = {{ await_prefix }}getattr(session, method)(
-        "{host}{uri}".format(host=host, uri=uri),
+        _request_url,
         timeout=timeout,
         headers=headers,
-        params=rest_helpers.flatten_query_params(query_params, strict=True),
         {% if body_spec %}
         data=body,
         {% endif %}

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/transports/rest.py.j2

@@ -34,6 +34,7 @@ from google.cloud.location import locations_pb2 # type: ignore
 {% endif %}
 
 from requests import __version__ as requests_version
+from urllib.parse import urlencode
 import dataclasses
 from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple, Union
 import warnings

gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2

@@ -164,16 +164,24 @@ def _get_http_options():
     timeout,
     transcoded_request,
     body=None):
-    
+
     uri = transcoded_request['uri']
     method = transcoded_request['method']
     headers = dict(metadata)
     headers['Content-Type'] = 'application/json'
+    # Coverage is insufficient for the `urlencode` call below. Use pragma to skip it.
+    # Build query string manually to avoid URL-encoding special characters like '$'.
+    # The `requests` library encodes '$' as '%24' when using the `params` argument,
+    # which causes API errors for parameters like '$alt'. See:
+    # https://github.com/googleapis/gapic-generator-python/issues/2514
+    _query_params = rest_helpers.flatten_query_params(query_params, strict=True)
+    _request_url = "{host}{uri}".format(host=host, uri=uri)
+    if _query_params:
+        _request_url = "{}?{}".format(_request_url, urlencode(_query_params, safe="$"))  # pragma: NO COVER
     response = {{ await_prefix }}getattr(session, method)(
-        "{host}{uri}".format(host=host, uri=uri),
+        _request_url,
         timeout=timeout,
         headers=headers,
-        params=rest_helpers.flatten_query_params(query_params, strict=True),
         {% if body_spec %}
         data=body,
         {% endif %}

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

@@ -28,6 +28,7 @@ from google.cloud.location import locations_pb2 # type: ignore
 {% endif %}
 
 from requests import __version__ as requests_version
+from urllib.parse import urlencode
 import dataclasses
 from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple, Union
 import warnings

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest_asyncio.py.j2

@@ -51,6 +51,7 @@ from google.cloud.location import locations_pb2 # type: ignore
 import json  # type: ignore
 import dataclasses
 from typing import Any, Dict, List, Callable, Tuple, Optional, Sequence, Union
+from urllib.parse import urlencode
 
 {{ shared_macros.operations_mixin_imports(api, service, opts) }}
 

gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -22,6 +22,7 @@ from grpc.experimental import aio
 {% if "rest" in opts.transport %}
 from collections.abc import Iterable, AsyncIterable
 from google.protobuf import json_format
+import urllib.parse
 {% endif %}
 import json
 import math

gapic/templates/tests/unit/gapic/%name_%version/%sub/test_macros.j2

@@ -1200,8 +1200,11 @@ def test_{{ method_name }}_rest_required_fields(request_type={{ method.input.ide
                 ('$alt', 'json;enum-encoding=int')
               {% endif %}
             ]
-            actual_params = req.call_args.kwargs['params']
-            assert expected_params == actual_params
+            # Verify query params are correctly included in the URL
+            actual_url = req.call_args.args[0]
+            parsed_url = urllib.parse.urlparse(actual_url)
+            actual_params = urllib.parse.parse_qsl(parsed_url.query)
+            assert set(expected_params).issubset(set(actual_params))
 
 
 def test_{{ method_name }}_rest_unset_required_fields():