feat(cerbero): HTTP client with bearer + bot-tag + retry

Client minimale verso Cerbero MCP unified server con header
Authorization Bearer e X-Bot-Tag su ogni richiesta. Retry esponenziale
solo su ConnectionError (non su 4xx). 4xx/5xx -> RuntimeError.

Switch da httpx a requests per allineamento con libreria mock
`responses` (httpx richiederebbe respx). httpx resta in deps per usi
futuri. Aggiunto types-requests ai dev deps per mypy strict.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     "anthropic>=0.39",
     "openai>=1.55",
     "httpx>=0.28",
+    "requests>=2.32",
     "tenacity>=9.0",
     "pyyaml>=6.0",
     "streamlit>=1.40",
@@ -31,6 +32,7 @@ dev = [
     "responses>=0.25",
     "ruff>=0.7",
     "mypy>=1.13",
+    "types-requests>=2.32",
 ]
 
 [build-system]