🐛[RUMF-1517] Remove specHelper export in src code

[bcaudan]

Motivation

Avoid to bloat customer packages with test code

Changes

• remove useless observers.specHelper exports
• add eslint rule to avoid this kind of issue

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #2126 (963a398) into main (4ad4ba4) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2126      +/-   ##
==========================================
+ Coverage   93.56%   93.58%   +0.01%     
==========================================
  Files         179      179              
  Lines        5985     5985              
  Branches     1342     1342              
==========================================
+ Hits         5600     5601       +1     
+ Misses        385      384       -1     

see 2 files with indirect coverage changes

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