feat: add explicit __enter__ and __aenter__ to BaseClient

[Ghraven]

Fixes #642

Problem

async with AsyncClient() as c: raises TypeError at runtime:

TypeError: object AsyncClient can't be used in 'async with' statement

BaseClient inherits AbstractContextManager and AbstractAsyncContextManager but only defined __exit__ and __aexit__. AbstractContextManager provides a default __enter__ returning self, but AbstractAsyncContextManager.__aenter__ is abstract and raises TypeError if not implemented.

Fix

Add explicit __enter__ and __aenter__ methods to BaseClient, both returning self, completing the context manager protocol for both sync and async clients.

Usage after this change

# Sync
with Client() as client:
    response = client.chat(...)

# Async
async with AsyncClient() as client:
    response = await client.chat(...)

Notes

• No behaviour change to __exit__ / __aexit__ — they still call close() / await close() as before.
• Client.__enter__ returns BaseClient — callers using the context manager with type annotations may want to cast to Client / AsyncClient if needed. A follow-up could tighten the return type using generics.