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

docs/source/providers/batches/index.md

@@ -2,12 +2,15 @@
 
 ## Overview
 
-Protocol for batch processing API operations.
-
-    The Batches API enables efficient processing of multiple requests in a single operation,
+The Batches API enables efficient processing of multiple requests in a single operation,
     particularly useful for processing large datasets, batch evaluation workflows, and
     cost-effective inference at scale.
 
+    The API is designed to allow use of openai client libraries for seamless integration.
+
+    This API provides the following extensions:
+     - idempotent batch creation
+
     Note: This API is currently under active development and may undergo changes.
 
 This section contains documentation for all available providers for the **batches** API.