[Inference API] Add unified api for chat completions (elastic#117589)

* Adding some shell classes

* modeling the request objects

* Writeable changes to schema

* Working parsing tests

* Creating a new action

* Add outbound request writing (WIP)

* Improvements to request serialization

* Adding separate transport classes

* separate out unified request and combine inputs

* Reworking unified inputs

* Adding unsupported operation calls

* Fixing parsing logic

* get the build working

* Update docs/changelog/117589.yaml

* Fixing injection issue

* Allowing model to be overridden but not working yet

* Fixing issues

* Switch field name for tool

* Add suport for toolCalls and refusal in streaming completion

* Working tool call response

* Separate unified and legacy code paths

* Updated the parser, but there are some class cast exceptions to fix

* Refactoring tests and request entities

* Parse response from OpenAI

* Removing unused request classes

* precommit

* Adding tests for UnifiedCompletionAction Request

* Refactoring stop to be a list of strings

* Testing for OpenAI response parsing

* Refactoring transport action tests to test unified validation code

* Fixing various tests

* Fixing license header

* Reformat streaming results

* Finalize response format

* remove debug logs

* remove changes for debugging

* Task type and base inference action tests

* Adding openai service tests

* Adding model tests

* tests for StreamingUnifiedChatCompletionResultsTests toXContentChunked

* Fixing change log and removing commented out code

* Switch usage to accept null

* Adding test for TestStreamingCompletionServiceExtension

* Avoid serializing empty lists + request entity tests

* Register named writeables from UnifiedCompletionRequest

* Removing commented code

* Clean up and add more of an explination

* remove duplicate test

* remove old todos

* Refactoring some duplication

* Adding javadoc

* Addressing feedback

---------

Co-authored-by: Jonathan Buttner <[email protected]>
Co-authored-by: Jonathan Buttner <[email protected]>
(cherry picked from commit 467fdb8)

# Conflicts:
#	x-pack/plugin/inference/qa/inference-service-tests/src/javaRestTest/java/org/elasticsearch/xpack/inference/InferenceCrudIT.java
#	x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/action/TransportInferenceAction.java
#	x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/common/DelegatingProcessor.java
#	x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/action/TransportInferenceActionTests.java

Author: maxhniebergall

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/action/TransportInferenceActionTests.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.action;
+
+import org.elasticsearch.action.support.ActionFilters;
+import org.elasticsearch.inference.InferenceServiceRegistry;
+import org.elasticsearch.transport.TransportService;
+import org.elasticsearch.xpack.core.inference.action.InferenceAction;
+import org.elasticsearch.xpack.inference.action.task.StreamingTaskManager;
+import org.elasticsearch.xpack.inference.registry.ModelRegistry;
+import org.elasticsearch.xpack.inference.telemetry.InferenceStats;
+
+import static org.mockito.Mockito.mock;
+
+public class TransportInferenceActionTests extends BaseTransportInferenceActionTestCase<InferenceAction.Request> {
+
+    @Override
+    protected BaseTransportInferenceAction<InferenceAction.Request> createAction(
+        TransportService transportService,
+        ActionFilters actionFilters,
+        ModelRegistry modelRegistry,
+        InferenceServiceRegistry serviceRegistry,
+        InferenceStats inferenceStats,
+        StreamingTaskManager streamingTaskManager
+    ) {
+        return new TransportInferenceAction(
+            transportService,
+            actionFilters,
+            modelRegistry,
+            serviceRegistry,
+            inferenceStats,
+            streamingTaskManager
+        );
+    }
+
+    @Override
+    protected InferenceAction.Request createRequest() {
+        return mock();
+    }
+}