mcp: align resource not found error code with sep-2164

[yarolegovich]

Change the error code returned by ResourceNotFoundError from -32002 to -32602 (jsonrpc.CodeInvalidParams), aligning with SEP-2164.

• Not gated with a protocol version check, because the previous error code was never formally specified and the behaviour varied across SDKs.
• In case clients which rely on the old error code exist, the previous behaviour can be restored with MCPGODEBUG=customresnotfounderrcode=1.
• CodeResourceNotFound deprecated since its only purpose going forward is backward compatibility. The new code should use jsonrpc.CodeInvalidParams directly.
• CodeResourceNotFound changed to var so it can stay in sync with ResourceNotFoundError and existing code doing code == mcp.CodeResourceNotFound keeps working in both the default and compat modes.