improvements(ai): Improve AI streaming UI/UX interactions + better separation of AI provider responsibilities (#2039)

* Start refactor

* Interface updates

* Rework Assistant, Provider, and tests for better domain boundaries

* Consolidate and simplify OpenAI provider and provider concepts

* Clean up assistant streaming

* Improve assistant message orchestration logic

* Clean up "thinking" UI interactions

* Remove stale class

* Regenerate VCR test responses

app/models/assistant/function_tool_caller.rb

@@ -0,0 +1,37 @@
+class Assistant::FunctionToolCaller
+  Error = Class.new(StandardError)
+  FunctionExecutionError = Class.new(Error)
+
+  attr_reader :functions
+
+  def initialize(functions = [])
+    @functions = functions
+  end
+
+  def fulfill_requests(function_requests)
+    function_requests.map do |function_request|
+      result = execute(function_request)
+
+      ToolCall::Function.from_function_request(function_request, result)
+    end
+  end
+
+  def function_definitions
+    functions.map(&:to_definition)
+  end
+
+  private
+    def execute(function_request)
+      fn = find_function(function_request)
+      fn_args = JSON.parse(function_request.function_args)
+      fn.call(fn_args)
+    rescue => e
+      raise FunctionExecutionError.new(
+        "Error calling function #{fn.name} with arguments #{fn_args}: #{e.message}"
+      )
+    end
+
+    def find_function(function_request)
+      functions.find { |f| f.name == function_request.function_name }
+    end
+end