[code cleanup] fix handleGetPayload to save every payload once the signature is verified

[michaelneuder]

📝 Summary

Previously, the payloads were only saved by the goroutine at the end of the function if the publication was successful. This caused an issue where we didn't save a delivered payload despite receiving a valid signature from the proposer.

I modified the function to be deferred immediately after the signature is successfully verified.

⛱ Motivation and Context

Once the signature is verified, the payload should be saved no matter what. Even if the publication fails for whatever reason, we want to store the cryptographic evidence that the proposer signed the header.

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #512 (3462896) into main (13ede49) will increase coverage by 0.01%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   31.70%   31.72%   +0.01%     
==========================================
  Files          24       24              
  Lines        4860     4858       -2     
==========================================
  Hits         1541     1541              
+ Misses       3127     3125       -2     
  Partials      192      192              

Flag	Coverage Δ
unittests	31.72% <0.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
services/api/service.go	37.91% <0.00%> (+0.04%)	⬆️