Prompt arguments cross the wire as strings, but the three trip prompts
declared tripId as z.number(), so every prompts/get failed with -32602
"expected number, received string", MCP Inspector included. The id is now
parsed from the string through a shared tripIdPromptArg (still an integer
in the safe range), PromptOptions only admits string-input schemas so the
mistake cannot come back unnoticed, and a prompt declared with an empty
argsSchema no longer demands an empty arguments object from the client.
The prompt tests run through a real in-memory MCP client instead of
calling the callbacks directly.