🐛[RUMF-1423] prevent unexpected behavior when our xhr are reused

[bcaudan]

Motivation

Following #1790, #1793, our new request strategy can be disturb if our xhr instances are reused to send other requests.

Changes

Prevent our xhr instances callback to be called multiple times.

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1797 (1d83412) into main (43ebf59) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1797      +/-   ##
==========================================
+ Coverage   90.95%   90.98%   +0.03%     
==========================================
  Files         129      129              
  Lines        5042     5037       -5     
  Branches     1134     1132       -2     
==========================================
- Hits         4586     4583       -3     
+ Misses        456      454       -2     

Impacted Files	Coverage Δ
...ckages/core/src/transport/sendWithRetryStrategy.ts	100.00% <ø> (+2.22%)	⬆️
packages/core/src/transport/httpRequest.ts	97.82% <100.00%> (+0.04%)	⬆️

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