Add plumbing to set instructions on Server

[glaciotech]

Added plumbing to allow setting Server level instructions and have it returned in the initialize result.

Motivation and Context

Allows for higher level instructions and context to be given about the server rather then just individual tools. For instance describing what the tools do and how they should be used.

How Has This Been Tested?

Yes, i tested this with Windsurf, for a MCP i'm developing. I checked it could see the instructions by asking it to summarize them. In addition I checked when Cascade was given a higher level instruction it invoked the tools in order as described in the Server instructions.

Breaking Changes

No. Instructions defaults to nil in the Server initialization which means by default it will perform the same as the previous code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [ x] 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
• [ N/A] I have added appropriate error handling
• [ N/A] I have added or updated documentation as needed

Additional context