refactor: remove unnecessary external-id from tool-call state

The external-id (thought signature for Google Gemini) doesn't need to
be stored in the tool-call state because:

1. The tool-call object from the LLM provider already contains it
2. When adding to history, we use the original tool-call object which
   preserves the external-id
3. The tool-call state is only for tracking execution status, not for
   reconstructing tool-calls

This addresses review feedback on PR #234 about why external-id was
stored in the state.

🤖 Generated with [eca](https://eca.dev)

Co-Authored-By: eca <[email protected]>

Author: ArthurHeymans

src/eca/features/chat.clj

@@ -560,7 +560,6 @@
            :server (:server event-data)
            :arguments (:arguments event-data)
            :origin (:origin event-data)
-           :external-id (:external-id event-data)
            :decision-reason {:code :none
                              :text "No reason"})
 
@@ -1125,7 +1124,7 @@
                                      :finish (do (add-to-history! {:role "assistant"
                                                                    :content [{:type :text :text @received-msgs*}]})
                                                  (finish-chat-prompt! :idle chat-ctx))))
-            :on-prepare-tool-call (fn [{:keys [id full-name arguments-text external-id]}]
+            :on-prepare-tool-call (fn [{:keys [id full-name arguments-text]}]
                                     (assert-chat-not-stopped! chat-ctx)
                                     (let [tool (tool-by-full-name full-name all-tools)]
                                       (transition-tool-call! db* chat-ctx id :tool-prepare
@@ -1134,7 +1133,6 @@
                                                               :full-name full-name
                                                               :origin (:origin tool)
                                                               :arguments-text arguments-text
-                                                              :external-id external-id
                                                               :summary (f.tools/tool-call-summary all-tools full-name nil config)})))
             :on-tools-called (on-tools-called! chat-ctx received-msgs* add-to-history!)
             :on-reason (fn [{:keys [status id text external-id]}]

test/eca/features/chat_tool_call_state_test.clj

@@ -35,7 +35,6 @@
               :server "eca"
               :arguments tool-arguments
               :origin tool-origin
-              :external-id nil
               :decision-reason {:code :none
                                 :text "No reason"}}
              (#'f.chat/get-tool-call-state @db* chat-id tool-call-id))