Add Google Imagen and Google Cloud TTS provider tools

Two new provider tools following the BaseTool pattern with auto-discovery:
- google_imagen: Imagen 4 image generation via Generative Language REST API
- google_tts: Google Cloud TTS with 700+ voices across 50+ languages

Both share GOOGLE_API_KEY env var. Selectors auto-discover them — no
selector code changes needed. Docs and contract tests updated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -5,6 +5,10 @@
 FAL_KEY=                     # FLUX images, Google Veo video, Kling video, MiniMax video, Recraft images
                              # Get one at https://fal.ai/dashboard/keys
 
+# --- Google (one key unlocks image gen + TTS) ---
+GOOGLE_API_KEY=              # Google Imagen images, Google Cloud TTS (700+ voices, 50+ languages)
+                             # Get one at https://aistudio.google.com/apikey
+
 # --- Voice ---
 ELEVENLABS_API_KEY=          # TTS narration, music generation, sound effects
 OPENAI_API_KEY=              # OpenAI TTS fallback and DALL-E image generation