feat(api): add non-streaming keep-alive support for idle timeout prevention

- Introduced `StartNonStreamingKeepAlive` to emit periodic blank lines during non-streaming responses.
- Added `nonstream-keepalive` configuration option in `SDKConfig`.
- Updated handlers to utilize `StartNonStreamingKeepAlive` and ensure proper cleanup.
- Extended config diff and tests to include `nonstream-keepalive` changes.

internal/watcher/diff/config_diff_test.go

@@ -231,10 +231,11 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 		AmpCode:                config.AmpCode{UpstreamAPIKey: "keep", RestrictManagementToLocalhost: false},
 		RemoteManagement:       config.RemoteManagement{DisableControlPanel: false, PanelGitHubRepository: "old/repo", SecretKey: "keep"},
 		SDKConfig: sdkconfig.SDKConfig{
-			RequestLog:       false,
-			ProxyURL:         "http://old-proxy",
-			APIKeys:          []string{"key-1"},
-			ForceModelPrefix: false,
+			RequestLog:         false,
+			ProxyURL:           "http://old-proxy",
+			APIKeys:            []string{"key-1"},
+			ForceModelPrefix:   false,
+			NonStreamKeepAlive: false,
 		},
 	}
 	newCfg := &config.Config{
@@ -267,10 +268,11 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 			SecretKey:             "",
 		},
 		SDKConfig: sdkconfig.SDKConfig{
-			RequestLog:       true,
-			ProxyURL:         "http://new-proxy",
-			APIKeys:          []string{" key-1 ", "key-2"},
-			ForceModelPrefix: true,
+			RequestLog:         true,
+			ProxyURL:           "http://new-proxy",
+			APIKeys:            []string{" key-1 ", "key-2"},
+			ForceModelPrefix:   true,
+			NonStreamKeepAlive: true,
 		},
 	}
 
@@ -285,6 +287,7 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 	expectContains(t, details, "proxy-url: http://old-proxy -> http://new-proxy")
 	expectContains(t, details, "ws-auth: false -> true")
 	expectContains(t, details, "force-model-prefix: false -> true")
+	expectContains(t, details, "nonstream-keepalive: false -> true")
 	expectContains(t, details, "quota-exceeded.switch-project: false -> true")
 	expectContains(t, details, "quota-exceeded.switch-preview-model: false -> true")
 	expectContains(t, details, "api-keys count: 1 -> 2")