cli: Fix datafusion-cli hint edge cases

[comphead]

Which issue does this PR close?

• Closes #.

Rationale for this change

The hint show condition relied on just checking if the input string is empty which caused visual issues like

What changes are included in this PR?

  helper.rs:                                                
  - Added a show_hint: Cell<bool> field to CliHelper. Uses Cell for interior mutability since hint() takes &self.                                                                                          
  - hint() now sets show_hint to false the moment line is non-empty. The hint only displays when show_hint is still true and the line is empty — i.e., only on the fresh prompt before any key is pressed. 
  - validate() calls reset_hint() after validation so the hint reappears on the next prompt after submission.                                                                                              
  - Added pub fn reset_hint(&self) for external reset.                                                                                                                                                     
                                                            
  exec.rs:
  - On Ctrl+C (ReadlineError::Interrupted), calls rl.helper().unwrap().reset_hint() so the next prompt still shows the hint.

Are these changes tested?

Are there any user-facing changes?

[comphead]

its just simplification