Feat/add context manager support

[SaiDevaHarshaR]

What this does

Fixes #614 — adds context manager support to Client and AsyncClient,
enabling the with-as syntax for automatic resource cleanup.

Usage

# Before — manual management
client = ollama.Client()
response = client.chat(...)

# After — automatic cleanup
with ollama.Client() as client:
    response = client.chat(...)
# client automatically cleaned up on exit

Changes

• Added __enter__ method to Client returning self
• Added __enter__ method to AsyncClient returning self
• __exit__ already existed on both classes calling self.close()

Testing

Verified with-as syntax works correctly and client
is cleaned up after the block exits.

Closes #614