fix: stop leaking exception messages to clients via JSON-RPC error data

Tool handler exceptions and anything reaching JsonRpcHandler's blind
rescue had their #message echoed into the error data member, exposing
internals to untrusted clients (CWE-209). Capability errors keep their
client-facing message by re-raising through RequestHandlerError, the
one channel whose message is deliberately surfaced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/mcp/server_context_test.rb

@@ -545,7 +545,7 @@ module MCP
       assert_nil response[:result]
       assert_equal(-32603, response[:error][:code])
       assert_equal "Internal error", response[:error][:message]
-      assert_match(/Internal error calling tool tool_with_required_context: /, response[:error][:data])
+      assert_equal "Internal error calling tool tool_with_required_context", response[:error][:data]
     end
 
     test "call_tool_with_args correctly detects server_context parameter presence" do