[MM-55264] Prevent call post from being modified

[streamer45]

Summary

PR prevents the call post from being modified as it could cause issues like messing with metadata such as recording file ids.

More context in the original comment #549 (comment)

Ticket Link

https://mattermost.atlassian.net/browse/MM-55264

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (1d769ec) 7.85% compared to head (22e976d) 7.84%.

Files	Patch %	Lines
server/plugin.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #608      +/-   ##
========================================
- Coverage   7.85%   7.84%   -0.01%     
========================================
  Files         26      26              
  Lines       5217    5223       +6     
========================================
  Hits         410     410              
- Misses      4762    4768       +6     
  Partials      45      45              

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