[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

docs/changelog/117589.yaml

@@ -0,0 +1,5 @@
+pr: 117589
+summary: "Add Inference Unified API for chat completions for OpenAI"
+area: Machine Learning
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/common/xcontent/ChunkedToXContentHelper.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.common.collect.Iterators;
 import org.elasticsearch.xcontent.ToXContent;
 
+import java.util.Collections;
 import java.util.Iterator;
 
 public enum ChunkedToXContentHelper {
@@ -53,6 +54,14 @@ public static Iterator<ToXContent> field(String name, String value) {
         return Iterators.single(((builder, params) -> builder.field(name, value)));
     }
 
+    public static Iterator<ToXContent> optionalField(String name, String value) {
+        if (value == null) {
+            return Collections.emptyIterator();
+        } else {
+            return field(name, value);
+        }
+    }
+
     /**
      * Creates an Iterator of a single ToXContent object that serializes the given object as a single chunk. Just wraps {@link
      * Iterators#single}, but still useful because it avoids any type ambiguity.

server/src/main/java/org/elasticsearch/inference/InferenceService.java

@@ -112,6 +112,23 @@ void infer(
     );
 
     /**
+     * Perform completion inference on the model using the unified schema.
+     *
+     * @param model        The model
+     * @param request Parameters for the request
+     * @param timeout      The timeout for the request
+     * @param listener     Inference result listener
+     */
+    void unifiedCompletionInfer(
+        Model model,
+        UnifiedCompletionRequest request,
+        TimeValue timeout,
+        ActionListener<InferenceServiceResults> listener
+    );
+
+    /**
+     * Chunk long text.
+     *
      * @param model           The model
      * @param query           Inference query, mainly for re-ranking
      * @param input           Inference input

server/src/main/java/org/elasticsearch/inference/TaskType.java

@@ -38,6 +38,10 @@ public static TaskType fromString(String name) {
     }
 
     public static TaskType fromStringOrStatusException(String name) {
+        if (name == null) {
+            throw new ElasticsearchStatusException("Task type must not be null", RestStatus.BAD_REQUEST);
+        }
+
         try {
             TaskType taskType = TaskType.fromString(name);
             return Objects.requireNonNull(taskType);