Relax Tool::call future bound to Send - #337

Fixes #291.

• Drop + Sync from the future returned by Tool::call (and ToolHolder::call); keep + Send.
• The Tool: Send + Sync bound on the implementor is unchanged.
• Rationale: Coordinator awaits tool calls sequentially, so Send alone is sufficient. The previous + Sync bound forced every .await inside a tool body to yield only Sync futures, ruling out common ecosystem types like bollard::Docker::start_exec, which returns Pin<Box<dyn Stream<…> + Send>> (not Sync).
• Non-breaking: relaxing an auto-trait bound on a return type is backwards-compatible — existing Tool impls continue to compile.
• No test added — this is a bound relaxation with no runtime behavior change, and the repo has no trybuild scaffolding for compile-only checks.