fix: emit success: false on ToolResultEvent for handled tool errors

[chloe-pomegranate]

Description

• Add a ToolError value object that Tool::handleToolException() returns instead of a plain string, allowing CallsTools::executeToolCall() to distinguish handled failures from genuine successes
• ToolResultEvent now correctly emits success: false with the error field populated when the default or custom error handler catches an exception

Closes #909

Testing

• Default error handler (validation error) produces ToolResultEvent(success: false)
• Default error handler (runtime error) produces ToolResultEvent(success: false)
• Custom error handler produces ToolResultEvent(success: false)
• withoutErrorHandling() still throws and produces ToolResultEvent(success: false) via PrismException
• Successful tool calls still produce ToolResultEvent(success: true)