* fix(rmcp-macros): use re-exported serde_json path in task_handler
Replace bare `::serde_json::` with `::rmcp::serde_json::` in
task_handler.rs to prevent compilation errors in crates that don't
directly depend on serde_json.
Fixes#487
* Update crates/rmcp-macros/src/task_handler.rs
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Co-authored-by: Dale Seo <5466341+DaleSeo@users.noreply.github.com>