fix: don't allow replying on closed tickets

[shadrak98]

Problem:

Customers/ Users were able to reply and thus re-open the tickets which were closed.

Fix:

Users won't be able to reply on the ticket if its marked as closed.

Screenshot:

On Resolved Status

On Closed Status

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (2d07252) 2.51% compared to head (c12402f) 2.56%.
Report is 22 commits behind head on main.

Files	Patch %	Lines
helpdesk/helpdesk/doctype/hd_ticket/hd_ticket.py	0.00%	21 Missing ⚠️
helpdesk/helpdesk/doctype/hd_ticket/api.py	0.00%	10 Missing ⚠️
...vice_level_agreement/hd_service_level_agreement.py	0.00%	3 Missing ⚠️
helpdesk/utils.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1642      +/-   ##
========================================
+ Coverage   2.51%   2.56%   +0.04%     
========================================
  Files         59      59              
  Lines       1908    1912       +4     
========================================
+ Hits          48      49       +1     
- Misses      1860    1863       +3     

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