feat: Add optional idempotency support to batches API

Implements optional idempotency for batch creation using `idem_tok` parameter:

* **Core idempotency**: Same token + parameters returns existing batch
* **Conflict detection**: Same token + different parameters raises HTTP 409 ConflictError
* **Metadata order independence**: Different key ordering doesn't affect idempotency

**API changes:**
- Add optional `idem_tok` parameter to `create_batch()` method
- Enhanced API documentation with idempotency extensions

**Implementation:**
- Reference provider supports idempotent batch creation
- ConflictError for proper HTTP 409 status code mapping
- Comprehensive parameter validation

**Testing:**
- Unit tests: focused tests covering core scenarios with parametrized conflict detection
- Integration tests: tests validating real OpenAI client behavior

This enables client-side retry safety and prevents duplicate batch creation
when using the same idempotency token, following REST API

tests/integration/batches/test_batches_idempotency.py

@@ -0,0 +1,91 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""
+Integration tests for batch idempotency functionality using the OpenAI client library.
+
+This module tests the idempotency feature in the batches API using the OpenAI-compatible
+client interface. These tests verify that the idempotency token (idem_tok) works correctly
+in a real client-server environment.
+
+Test Categories:
+1. Successful Idempotency: Same token returns same batch with identical parameters
+   - test_idempotent_batch_creation_successful: Verifies that requests with the same
+     idempotency token return identical batches, even with different metadata order
+
+2. Conflict Detection: Same token with conflicting parameters raises HTTP 409 errors
+   - test_idempotency_conflict_with_different_params: Verifies that reusing an idempotency token
+     with truly conflicting parameters (both file ID and metadata values) raises ConflictError
+"""
+
+import time
+
+import pytest
+from openai import ConflictError
+
+
+class TestBatchesIdempotencyIntegration:
+    """Integration tests for batch idempotency using OpenAI client."""
+
+    def test_idempotent_batch_creation_successful(self, openai_client):
+        """Test that identical requests with same idempotency token return the same batch."""
+        batch1 = openai_client.batches.create(
+            input_file_id="bogus-id",
+            endpoint="/v1/chat/completions",
+            completion_window="24h",
+            metadata={
+                "test_type": "idempotency_success",
+                "purpose": "integration_test",
+            },
+            extra_body={"idem_tok": "test-idempotency-token-1"},
+        )
+
+        # sleep to ensure different timestamps
+        time.sleep(1)
+
+        batch2 = openai_client.batches.create(
+            input_file_id="bogus-id",
+            endpoint="/v1/chat/completions",
+            completion_window="24h",
+            metadata={
+                "purpose": "integration_test",
+                "test_type": "idempotency_success",
+            },  # Different order
+            extra_body={"idem_tok": "test-idempotency-token-1"},
+        )
+
+        assert batch1.id == batch2.id
+        assert batch1.input_file_id == batch2.input_file_id
+        assert batch1.endpoint == batch2.endpoint
+        assert batch1.completion_window == batch2.completion_window
+        assert batch1.metadata == batch2.metadata
+        assert batch1.created_at == batch2.created_at
+
+    def test_idempotency_conflict_with_different_params(self, openai_client):
+        """Test that using same idempotency token with different params raises conflict error."""
+        batch1 = openai_client.batches.create(
+            input_file_id="bogus-id-1",
+            endpoint="/v1/chat/completions",
+            completion_window="24h",
+            metadata={"test_type": "conflict_test_1"},
+            extra_body={"idem_tok": "conflict-token"},
+        )
+
+        with pytest.raises(ConflictError) as exc_info:
+            openai_client.batches.create(
+                input_file_id="bogus-id-2",  # Different file ID
+                endpoint="/v1/chat/completions",
+                completion_window="24h",
+                metadata={"test_type": "conflict_test_2"},  # Different metadata
+                extra_body={"idem_tok": "conflict-token"},  # Same token
+            )
+
+        assert exc_info.value.status_code == 409
+        assert "conflict" in str(exc_info.value).lower()
+
+        retrieved_batch = openai_client.batches.retrieve(batch1.id)
+        assert retrieved_batch.id == batch1.id
+        assert retrieved_batch.input_file_id == "bogus-id-1"