ServerHandler defaults answer prompts/list and completion/complete when neither capability is declared #1445

In rmcp 3.9.0 (and on main), the default ServerHandler::list_prompts answers Ok(ListPromptsResult::default()) and the default complete answers Ok(CompleteResult::default()) (crates/rmcp/src/handler/server.rs), while get_prompt, set_level and subscribe default to method_not_found.

So a server whose get_info() declares neither the prompts nor the completions capability still answers prompts/list with {"prompts":[]} and completion/complete with {"completion":{"values":[]}}.

Why it matters

• Spec 2026-07-28, Prompts: servers that support prompts MUST declare the prompts capability; its error handling lists -32601 for a capability not supported.
• Spec 2025-11-25, Lifecycle: both parties MUST only use capabilities that were successfully negotiated.
• The conformance suite (0.2.0-alpha.12) check sep-2575-server-declares-prompts-in-discover fails such a server ("Server handles prompts but does not declare the prompts capability").

Suggestion

Default both to method_not_found unless the matching capability is declared in get_info(), as validate_tasks_capability already does for tasks. list_resources, list_resource_templates and list_tools have the same Ok(default) defaults and may deserve the same treatment.

Workaround

Override list_prompts and complete to return Err(McpError::method_not_found::<…>()).