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

[WeekendSuperhero]

Motivation and Context

Fixes #801

The #[tool] macro correctly omits + Send on generated handler futures when the local crate feature is enabled (tool.rs:341). The #[prompt] macro was not updated when local support was added in #740 — it unconditionally emits + Send on all generated futures (prompt.rs:141,144).

This makes it impossible to use !Send types in prompt handlers even with the local feature enabled, despite tool handlers working correctly. The real-world use case is MCP servers wrapping platform APIs with thread-affine types (e.g., Apple EventKit's EKEventStore).

How Has This Been Tested?

• Existing tests pass locally (cargo test --workspace)
• Verified that a server struct holding Rc<String> (!Send) with both #[tool] and #[prompt] handlers compiles with features = ["local"] after this change (previously only #[tool] compiled)

Breaking Changes

None. This only changes behavior when the local feature is explicitly enabled. Default behavior (without local) is unchanged — + Send is still emitted.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The fix mirrors the existing pattern in tool.rs:339-341:

let omit_send = cfg!(feature = "local") || attribute.local;

Applied the same cfg!(feature = "local") check to prompt.rs for both ReturnType::Default and ReturnType::Type match arms.

[DaleSeo]

omit_send isn't defined, so it won't compile!