Improve JSON-RPC error handling with typed errors and response constructors

[peteski22]

Description

Enhances JSON-RPC error handling by adding typed sentinel errors and response constructors to improve error checking patterns and reduce code duplication.

Summary:

• Adds sentinel errors (ErrMethodNotFound, ErrInvalidParams, etc.) with errors.Is() support
• Introduces AsError() method on JSONRPCErrorDetails for converting to typed errors
• Adds response constructors (NewJSONRPCErrorResponse, NewJSONRPCResultResponse) to eliminate repetitive struct construction

Closes: #593

Type of Change

• New feature (non-breaking change that adds functionality)
• Code refactoring (no functional changes)

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works

MCP Spec Compliance

• This PR implements a feature defined in the MCP specification
• Link to relevant spec section: Link text
• Implementation follows the specification exactly

Additional Information

This change enables cleaner error handling patterns in code which consumes the mcp-go library:

err := response.Error.AsError()
if errors.Is(err, mcp.ErrMethodNotFound) {
  // handle method not found
}

It also reduces repetitive response construction throughout the transport layer with dedicated constructors.