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>

tests/contracts/test_phase3_contracts.py

@@ -143,7 +143,7 @@ class TestCapabilityMetadata:
         catalog = reg.capability_catalog()
         assert "tts" in catalog
         providers = {item["provider"] for item in catalog["tts"] if item["provider"] != "selector"}
-        assert providers == {"elevenlabs", "openai", "piper"}
+        assert providers == {"elevenlabs", "google_tts", "openai", "piper"}
 
 
 # ---- Animated Explainer Pipeline ----