fix: respect proxy env vars in runner client (#962)

Fixes #957.

## Why
The runner builds a custom `http.Transport` for its RPC client. From first principles, once we stop using the default transport we also stop inheriting its default proxy resolution behavior, so `HTTP_PROXY`/`HTTPS_PROXY` are ignored unless we wire that behavior back explicitly.

## What
- set `Proxy: http.ProxyFromEnvironment` on the custom transport
- add a regression test that verifies `getHTTPClient` honors proxy environment variables

Reviewed-on: https://gitea.com/gitea/runner/pulls/962
Reviewed-by: ChristopherHX <38043+christopherhx@noreply.gitea.com>

internal/pkg/client/http_test.go

@@ -0,0 +1,27 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package client
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetHTTPClientUsesProxyFromEnvironment(t *testing.T) {
+	t.Setenv("HTTP_PROXY", "http://proxy.example.com:8080")
+
+	client := getHTTPClient("http://gitea.example.com", false)
+	transport, ok := client.Transport.(*http.Transport)
+	require.True(t, ok)
+
+	req, err := http.NewRequest(http.MethodGet, "http://gitea.example.com/api/actions/ping", nil)
+	require.NoError(t, err)
+
+	proxyURL, err := transport.Proxy(req)
+	require.NoError(t, err)
+	require.NotNil(t, proxyURL)
+	require.Equal(t, "http://proxy.example.com:8080", proxyURL.String())
+}