feat(llm): retry tenacity su errori transient connection/timeout/5xx

Avvolge LLMClient.complete con tenacity (3 attempts, backoff esponenziale
2-10s) che ritenta solo su errori transient di OpenAI/Anthropic SDK
(APIConnectionError, APITimeoutError, InternalServerError). RateLimit,
Authentication e 4xx non vengono ritentati. reraise=True preserva
l'eccezione originale dopo l'esaurimento dei tentativi.

Aggiunti 2 test (marker slow): esaurimento retry su APIConnectionError
e successo al secondo tentativo dopo APITimeoutError.

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

src/multi_swarm/llm/client.py

@@ -2,8 +2,16 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 
+import anthropic
+import openai
 from anthropic import Anthropic
 from openai import OpenAI
+from tenacity import (
+    retry,
+    retry_if_exception_type,
+    stop_after_attempt,
+    wait_exponential,
+)
 
 from ..genome.hypothesis import HypothesisAgentGenome, ModelTier
 
@@ -12,6 +20,16 @@ MODEL_TIER_C = "qwen/qwen-2.5-72b-instruct"  # via OpenRouter
 MODEL_TIER_B = "claude-sonnet-4-6"  # via Anthropic
 OPENROUTER_BASE_URL = "https://openrouter.ai/api/v1"
 
+# Errori transient: retry. RateLimit/Auth/InvalidRequest: NO retry.
+_RETRYABLE_EXCEPTIONS: tuple[type[BaseException], ...] = (
+    openai.APIConnectionError,
+    openai.APITimeoutError,
+    openai.InternalServerError,
+    anthropic.APIConnectionError,
+    anthropic.APITimeoutError,
+    anthropic.InternalServerError,
+)
+
 
 @dataclass(frozen=True)
 class CompletionResult:
@@ -31,6 +49,12 @@ class LLMClient:
         self._openrouter = OpenAI(api_key=openrouter_api_key, base_url=OPENROUTER_BASE_URL)
         self._anthropic = Anthropic(api_key=anthropic_api_key) if anthropic_api_key else None
 
+    @retry(
+        stop=stop_after_attempt(3),
+        wait=wait_exponential(multiplier=1.0, min=2.0, max=10.0),
+        retry=retry_if_exception_type(_RETRYABLE_EXCEPTIONS),
+        reraise=True,
+    )
     def complete(
         self,
         genome: HypothesisAgentGenome,