Expose sampling capabilities on high-level clients

[sohammmmm10]

Motivation and Context

How Has This Been Tested?

Breaking Changes

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

[sohammmmm10]

All CI checks are passing.

This PR keeps the change scoped to exposing the existing ClientSession.sampling_capabilities support through the high-level Client and ClientSessionGroup APIs, with interaction coverage for sampling.context and sampling.tools.

Happy to adjust scope if maintainers prefer separating API pass-through from the interaction-test updates.