client: accept Enum classes as structured output formats

[WitBlue6]

Summary

Allow a Python Enum class as the format argument for chat and generate, in both synchronous and asynchronous clients. The request model converts the enum to its JSON Schema with Pydantic TypeAdapter before serialization. Existing string and mapping formats remain unchanged.

Closes #496.

Tests

• Added request-level tests for sync/async chat and generate, asserting that the JSON sent to Ollama contains the enum schema.
• pytest tests/test_client.py -k "not connection_error": 76 passed, 2 deselected. The two existing localhost connection-error tests receive HTTP 502 from the current environment's proxy.
• ruff check and ruff format --check pass for all changed files.