allow multiple tool calls, log warnings if there are duplicate calls.

controlled via a flag to the agent config:
allow_multiple_tool_calls = true

crates/g3-config/Cargo.toml

@@ -15,3 +15,4 @@ dirs = "5.0"
 
 [dev-dependencies]
 tempfile = "3.8"
+serde_json = { workspace = true }