fix(macros): respect local feature in #[prompt] macro — omit + Send bound

[WeekendSuperhero]

Fixes modelcontextprotocol#801

The #[prompt] macro unconditionally emits + Send on generated handler futures, preventing !Send types from being used in prompt handlers even when the local crate feature is enabled.

This brings #[prompt] to parity with #[tool], which already checks cfg!(feature = "local") to conditionally omit the Send bound (tool.rs:341).

Changes

• Add let omit_send = cfg!(feature = "local"); in the prompt return type generation
• Gate + Send behind !omit_send for both ReturnType::Default and ReturnType::Type arms