feat(batches, completions): add /v1/completions support to /v1/batches (#3309)

# What does this PR do?

add support for /v1/completions to the /v1/batches api


## Test Plan

ci

tests/unit/providers/batches/test_reference.py

@@ -46,7 +46,8 @@ The tests are categorized and outlined below, keep this updated:
   * test_validate_input_url_mismatch (negative)
   * test_validate_input_multiple_errors_per_request (negative)
   * test_validate_input_invalid_request_format (negative)
-  * test_validate_input_missing_parameters (parametrized negative - custom_id, method, url, body, model, messages missing validation)
+  * test_validate_input_missing_parameters_chat_completions (parametrized negative - custom_id, method, url, body, model, messages missing validation for chat/completions)
+  * test_validate_input_missing_parameters_completions (parametrized negative - custom_id, method, url, body, model, prompt missing validation for completions)
   * test_validate_input_invalid_parameter_types (parametrized negative - custom_id, url, method, body, model, messages type validation)
 
 The tests use temporary SQLite databases for isolation and mock external
@@ -213,7 +214,6 @@ class TestReferenceBatchesImpl:
         "endpoint",
         [
             "/v1/embeddings",
-            "/v1/completions",
             "/v1/invalid/endpoint",
             "",
         ],
@@ -499,8 +499,10 @@ class TestReferenceBatchesImpl:
             ("messages", "body.messages", "invalid_request", "Messages parameter is required"),
         ],
     )
-    async def test_validate_input_missing_parameters(self, provider, param_name, param_path, error_code, error_message):
-        """Test _validate_input when file contains request with missing required parameters."""
+    async def test_validate_input_missing_parameters_chat_completions(
+        self, provider, param_name, param_path, error_code, error_message
+    ):
+        """Test _validate_input when file contains request with missing required parameters for chat completions."""
         provider.files_api.openai_retrieve_file = AsyncMock()
         mock_response = MagicMock()
 
@@ -541,6 +543,61 @@ class TestReferenceBatchesImpl:
         assert errors[0].message == error_message
         assert errors[0].param == param_path
 
+    @pytest.mark.parametrize(
+        "param_name,param_path,error_code,error_message",
+        [
+            ("custom_id", "custom_id", "missing_required_parameter", "Missing required parameter: custom_id"),
+            ("method", "method", "missing_required_parameter", "Missing required parameter: method"),
+            ("url", "url", "missing_required_parameter", "Missing required parameter: url"),
+            ("body", "body", "missing_required_parameter", "Missing required parameter: body"),
+            ("model", "body.model", "invalid_request", "Model parameter is required"),
+            ("prompt", "body.prompt", "invalid_request", "Prompt parameter is required"),
+        ],
+    )
+    async def test_validate_input_missing_parameters_completions(
+        self, provider, param_name, param_path, error_code, error_message
+    ):
+        """Test _validate_input when file contains request with missing required parameters for text completions."""
+        provider.files_api.openai_retrieve_file = AsyncMock()
+        mock_response = MagicMock()
+
+        base_request = {
+            "custom_id": "req-1",
+            "method": "POST",
+            "url": "/v1/completions",
+            "body": {"model": "test-model", "prompt": "Hello"},
+        }
+
+        # Remove the specific parameter being tested
+        if "." in param_path:
+            top_level, nested_param = param_path.split(".", 1)
+            del base_request[top_level][nested_param]
+        else:
+            del base_request[param_name]
+
+        mock_response.body = json.dumps(base_request).encode()
+        provider.files_api.openai_retrieve_file_content = AsyncMock(return_value=mock_response)
+
+        batch = BatchObject(
+            id="batch_test",
+            object="batch",
+            endpoint="/v1/completions",
+            input_file_id=f"missing_{param_name}_file",
+            completion_window="24h",
+            status="validating",
+            created_at=1234567890,
+        )
+
+        errors, requests = await provider._validate_input(batch)
+
+        assert len(errors) == 1
+        assert len(requests) == 0
+
+        assert errors[0].code == error_code
+        assert errors[0].line == 1
+        assert errors[0].message == error_message
+        assert errors[0].param == param_path
+
     async def test_validate_input_url_mismatch(self, provider):
         """Test _validate_input when file contains request with URL that doesn't match batch endpoint."""
         provider.files_api.openai_retrieve_file = AsyncMock()