fix: Prompt validation errors are logged with an unnecessary traceback (#3342) - #3412

modelcontextprotocol#3342)

When a prompt is invoked without a required argument, the request is
correctly rejected — but the server logs the resulting ValueError as a
full traceback at ERROR level via logger.exception(). For expected,
user-facing validation failures (missing required arguments) this
floods server logs with stack traces that operators have to wade
through.

Changes:
* Add PromptArgumentError(ValueError) to mcp.server.mcpserver.exceptions
  — stays a ValueError subclass for backward compatibility, but lets
  the server distinguish known user-input failures from unexpected bugs.
* In Prompt.render(), raise PromptArgumentError instead of bare ValueError.
* In MCPServer.get_prompt(), add an except clause that logs
  PromptArgumentError at WARNING without exc_info, then re-raises the
  same ValueError the protocol layer expects.
* Tests:
  - test_get_prompt_missing_args_logs_warning_no_traceback — verifies
    WARNING log is emitted and no ERROR/EXCEPTION log is produced for
    known validation errors.
  - test_prompt_argument_error_is_value_error_subclass — verifies
    backward compat: existing user code catching ValueError still
    catches the new exception.

22/22 prompt tests pass locally (21 pre-existing + 1 pre-existing test
plus my 2 new tests). All 21 existing prompt tests still pass.