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/src/handlers/ast.rs

@@ -504,7 +504,7 @@ impl McpTool for FindSymbolReferencesHandler {
             matches.extend(disk_matches);
         }
 
-        Ok(serde_json::to_string_pretty(&matches)?)
+        Ok(serde_yaml::to_string(&matches)?)
     }
 }
 
@@ -560,7 +560,7 @@ impl McpTool for GetCallersHandler {
             callers.extend(disk_callers);
         }
 
-        Ok(serde_json::to_string_pretty(&callers)?)
+        Ok(serde_yaml::to_string(&callers)?)
     }
 }
 
@@ -649,7 +649,7 @@ impl McpTool for AnalyzeImpactHandler {
             }
         });
 
-        Ok(serde_json::to_string_pretty(&result)?)
+        Ok(serde_yaml::to_string(&result)?)
     }
 }