feat(mcp): switch to YAML serialization for tool outputs

- Replaced serde_json with serde_yaml in all handler endpoints.

- This drastically reduces token consumption for large list/graph responses by removing JSON braces and quotes.

server/Cargo.toml

@@ -51,6 +51,7 @@ tokio-stream = { version = "0.1", features = ["sync"] }
 chrono = { version = "0.4.45", features = ["serde"] }
 ocrs = "0.13.1"
 rten = "0.26.0"
+serde_yaml = "0.9.34"
 
 [build-dependencies]
 chrono = "0.4.45"