Users frequently call callServerTool("tool_name", args) instead of the
correct callServerTool({ name: "tool_name", arguments: args }), resulting
in a silent/confusing failure. Detect this and throw immediately with
a message that shows the correct call shape.
Also fixes the incorrect example in examples/pdf-server/README.md that
demonstrated this same wrong usage pattern.
Fixes#386https://claude.ai/code/session_01GqAyN4Ux7svWoU2HqsSLZF
Co-authored-by: Claude <noreply@anthropic.com>