Fix: Preserve McpError semantics when prompt methods are invoked via reflection

[vinsguru]

Problem

Prompt methods are invoked using Method.invoke, which wraps user-thrown exceptions in InvocationTargetException.
Because of this, McpError thrown by prompt implementations is never detected via instanceof checks and gets overridden by a generic error, losing its JSON-RPC code, message, and data.

Root Cause

Reflection wraps the original exception. The existing catch block checks Exception instanceof McpError, which is guaranteed to fail when the error is thrown from the invoked method.

Solution

• Unwrap InvocationTargetException to obtain the real cause.
• Re-throw McpError unchanged when encountered.
• Convert all other exceptions into a structured McpError with a meaningful JSON-RPC error response.

Behavioral Changes

• McpError thrown by prompt methods now propagates correctly.
• Runtime exceptions from prompt methods are mapped to McpError instead of being swallowed or replaced.
• Error details are preserved and testable.

Tests

• Added/Updated coverage to verify:
  • Runtime exceptions are converted into McpError
  • Existing McpError instances are not overridden

This fixes #94