fix: restore original formatError usage in tool-executor

[cleyesode]

Summary

Restores original `formatError()` usage in the tool-executor to maintain consistency with the project's error handling pattern.

Problem

The original implementation removed `formatError()` from error messages, which:

• Created inconsistency with `message-handler.ts`
• Broke error handling for object errors without message properties
• Could cause issues with `displayToolResult()` which expects the "Error: " prefix

Solution

Restores `formatError()` import and usage for both validation and execution errors.

Changes

• Add back `formatError` import from `@/utils/error-formatter`
• Use `formatError()` for validation error messages: `Validation failed: ${formatError(error)}`
• Use `formatError()` for execution error messages: `Error: ${formatError(error)}`

Testing

All 10 existing tests continue to pass.

Files Changed

• `source/hooks/chat-handler/conversation/tool-executor.tsx` (+1, -1)

Related

• PR feat: implement parallel tool execution for better performance #349: feat: implement parallel tool execution for better performance
• Maintains consistency with existing error handling patterns