test: add tests for connection error handling

[Shreyas-Gowda26]

Closes #6
Added tests/test_connection_errors.py with pytest tests for connection failures:

• Server unreachable
• Connection timeout
• DNS resolution failure
• Client cleanup after failed connection
• Reconnection after failure
• Context manager handling connection errors

Uses unittest.mock to simulate network failures without needing a real server.

Closes #5

Added missing type annotations across the SDK to pass mypy --strict:

• exceptions.py: added return types and replaced str | None with Optional[str]
• types.py: added Iterator return type to iter and len
• client.py: typed **kwargs and aexit parameters
• producer.py: typed aexit parameters
• consumer.py: typed aexit and added explicit casts for aiokafka returns
• admin.py: typed aexit parameters
• serializers.py: added explicit casts and typed dict parameters
• retry.py: added Callable[..., Any] type parameters
• telemetry.py: added Callable[..., Any] type parameters

All 32 mypy --strict errors resolved.