Add close() and context manager support to Client and AsyncClient

[chassing]

Summary

Fixes #144

The sync Client had no way to close the underlying httpx connection pool, leaking file descriptors until process exit. The async AsyncClient had close() but lacked aclose() and async context manager support.

• Add close(), __enter__, __exit__ to sync Client
• Add aclose(), __aenter__, __aexit__ to AsyncClient
• Add close() to GenericSyncClient
• Update docs with a "Closing the client" section
• Update README examples to use context managers
• Bump version to 0.22.0

Usage

# Sync
with Client() as client:
    pod = client.get(Pod, name="my-pod")

# Async
async with AsyncClient() as client:
    pod = await client.get(Pod, name="my-pod")

Test plan

• Added test_close and test_context_manager for sync Client
• Added test_close, test_aclose, test_async_context_manager for AsyncClient
• Full test suite passes (189 tests)

[gtsystem]

CI is failing. Can you run uv lock and commit the changes to the lock file?