fix(hypothesis): catch EmptyCompletionError dentro propose() invece di propagare

Run phase2-5-qwen25-prompt-mut-001 fallito perché qwen-2.5-72b ha emesso
empty completion ripetutamente; le 3 retry tenacity in LLMClient.complete
si sono esaurite e l'exception è bollata fino a run_phase1, marcando l'intero
run come failed (perso ~$0.017 di tier C).

Fix: HypothesisAgent.propose() ora cattura EmptyCompletionError nel loop
max_attempts trattandola come parse-fail "empty_completion" e ritentando
con max_retries+1 tentativi (cumulativo: max_retries tenacity × max_attempts
loop esterno, default 3×3 = 9 retry effettive). Se TUTTI i tentativi
falliscono con empty, ritorna proposal con strategy=None e
parse_error="empty_completion: ...", lasciando l'orchestrator continuare
con quel genome marcato come "fitness=0" senza crash totale del run.

+2 test: success dopo 1 empty + retry; failure totale dopo 3 empty consecutive.

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

tests/unit/test_hypothesis_agent.py

@@ -2,7 +2,7 @@ import json
 
 from multi_swarm.agents.hypothesis import HypothesisAgent, MarketSummary
 from multi_swarm.genome.hypothesis import HypothesisAgentGenome, ModelTier
-from multi_swarm.llm.client import CompletionResult
+from multi_swarm.llm.client import CompletionResult, EmptyCompletionError
 
 
 def make_summary() -> MarketSummary:
@@ -214,3 +214,37 @@ def test_hypothesis_agent_no_retry_when_first_succeeds(mocker):  # type: ignore[
     assert proposal.n_attempts == 1
     assert len(proposal.completions) == 1
     assert fake_llm.complete.call_count == 1
+
+
+def test_hypothesis_agent_retries_on_empty_completion(mocker):  # type: ignore[no-untyped-def]
+    """LLMClient esaurisce retry tenacity → propose ritenta nel loop max_attempts."""
+    fake_llm = mocker.MagicMock()
+    fake_llm.complete.side_effect = [
+        EmptyCompletionError("empty response from qwen"),
+        CompletionResult(
+            text=VALID_STRATEGY_JSON,
+            input_tokens=200,
+            output_tokens=80,
+            tier=ModelTier.C,
+            model="qwen",
+        ),
+    ]
+    agent = HypothesisAgent(llm=fake_llm, max_retries=2)
+    proposal = agent.propose(make_genome(), make_summary())
+    assert proposal.strategy is not None
+    assert fake_llm.complete.call_count == 2
+    # n_attempts conta solo le completions arrivate (skipping empty failures).
+    assert len(proposal.completions) == 1
+
+
+def test_hypothesis_agent_returns_failed_proposal_on_only_empty_completions(mocker):  # type: ignore[no-untyped-def]
+    """Tutti i tentativi sollevano EmptyCompletionError → proposal con strategy None."""
+    fake_llm = mocker.MagicMock()
+    fake_llm.complete.side_effect = EmptyCompletionError("empty response")
+    agent = HypothesisAgent(llm=fake_llm, max_retries=2)
+    proposal = agent.propose(make_genome(), make_summary())
+    assert proposal.strategy is None
+    assert proposal.parse_error is not None
+    assert "empty_completion" in proposal.parse_error
+    # 3 tentativi tutti falliti.
+    assert fake_llm.complete.call_count == 3