fix(agents,backends): claude sessions read sdk messages up to 16 mib

src/beaver_gateway/agents/claude.py

@@ -46,6 +46,7 @@ class ClaudeOptions(BaseModel):
     """Extra inherited variable names to let through the env whitelist."""
 
     max_turns: int | None = None
+    max_buffer_size: int = 16 * 1024 * 1024
     session_store_flush: str = "eager"
     """``eager`` mirrors every transcript frame as it lands, so a gateway
     killed mid-turn loses at most the frame in flight."""