feat: add mcp client instruction support

[kalvinnchau]

The companion to #120, this adds instruction support on the client side.

Updates the types schema to include an optional instruction with a comment taken directly from the MCP schema spec at (https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.ts#L170-L174)

This adds a private getter to allow any applications building around the client to retrieve the instructions when needed.

Motivation and Context

Allows users to get instructions from an mcp client object if the server sends one.

How Has This Been Tested?

Updated tests for client to verify instructions when sent, and undefined when not.

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

[jspahrsummers]

Great, thank you!