feat(voice): harden Gemini Live provider integration

src/voice/providerCapabilities.js

@@ -0,0 +1,33 @@
+export const VOICE_PROVIDER_GUIDANCE =
+  'I can’t switch voice providers directly. Hover over the Voice panel and use the provider dropdown.';
+export const HOST_VOICE_PROVIDER_GUIDANCE =
+  'I can’t switch voice providers directly. Provider selection is managed by this app.';
+
+/** Host construction decides which controls exist; tool text cannot add one. */
+export function voiceProviderGuidance({ providerDropdown = false } = {}) {
+  return providerDropdown === true
+    ? VOICE_PROVIDER_GUIDANCE
+    : HOST_VOICE_PROVIDER_GUIDANCE;
+}
+
+export const VOICE_PROVIDERS = Object.freeze({
+  openai: Object.freeze({ label: 'OpenAI' }),
+  gemini: Object.freeze({ label: 'Gemini' }),
+});
+
+/** Optional providers require an explicit host opt-in and an available factory. */
+export function supportedVoiceProviders(
+  requested = ['openai'],
+  factories = {},
+) {
+  if (!Array.isArray(requested))
+    throw new TypeError('supportedProviders must be an array');
+  const selected = [...new Set(requested)].filter(
+    (name) =>
+      Object.hasOwn(VOICE_PROVIDERS, name) &&
+      typeof factories[name] === 'function',
+  );
+  if (!selected.length)
+    throw new Error('No supported voice provider is configured');
+  return Object.freeze(selected);
+}