Fix tool error handling to follow MCP spec

[Jflick58]

Tool execution errors now return successful responses with isError: true
instead of JSON-RPC protocol errors, per the MCP specification.

Motivation and Context

This allows clients to distinguish between protocol-level errors and
tool execution errors, providing better error context.

Fixes #159

How Has This Been Tested?

Updated unit tests to support new error behavior. All existing tests passing.

Breaking Changes

If client libs are parsing the error message for the specific tool call error, then they will likely get a parser error due to the change of the error structure.

Current behavior:

    {
      "jsonrpc": "2.0",
      "id": 4,
      "error": {
        "code": -32603,
        "message": "Internal error",
        "data": "Internal error calling tool fetch_weather_data"
      }
    }

Updated behavior (matches spec and readme):

     {
       "jsonrpc": "2.0",
       "id": 4,
       "result": {
         "content": [{
           "type": "text",
           "text": "Internal error calling tool fetch_weather_data"
         }],
         "isError": true
       }
     }

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
• [x ] I have added or updated documentation as needed

[koic]

I've left one small comment, but overall it looks good to me. Can you address it and squash the commits in to one?

[koic]

@Jflick58 ping.

[Jflick58]

@koic sorry have been sick. I'll get this buttoned up

[koic]

@Jflick58 Thank you for working on this!