[backend] Adding the content-type header when calling MistralAI

[Dimfacion]

Proposed changes

• Adding the Content-Type in the headers

Related issues

• Closes Issue when using MistralAI server #2063

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 33.43%. Comparing base (363ec66) to head (6ac4e27).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...src/main/java/io/openbas/rest/stream/ai/AiApi.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2064      +/-   ##
============================================
- Coverage     33.43%   33.43%   -0.01%     
  Complexity     1668     1668              
============================================
  Files           574      574              
  Lines         16799    16800       +1     
  Branches        992      992              
============================================
  Hits           5617     5617              
- Misses        10928    10929       +1     
  Partials        254      254              

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