Client sampling and roots capabilities set to None if not implemented

[lorenzocesconetto]

Right now clients always claims to support sampling and roots capabilities, which is inaccurate.
Having this properly set allows servers to fallback to alternative implementations.

Motivation and Context

Servers might rely on the correctness of client capabilities, so clients should inform them accurately.

How Has This Been Tested?

Unit tests

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[lorenzocesconetto]

@ihrpr I think this is an important PR to make sure the SDK adheres to the protocol.
This enables servers to make decisions based on the client capabilities.
For instance, server won't waste time trying to send a sampling request to a client that has no sampling capabilities.