fix(x/ecocredit): add missing transfer event

[ryanchristo]

Description

Ref: #1660

While auditing buy direct, I noticed we are missing a transfer event. This pull request adds the missing transfer event and also improves comments for the fill order logic used when purchasing credits.

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Author Checklist

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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #1667 (0a0f8e2) into main (728547a) will decrease coverage by 0.00%.
The diff coverage is 88.00%.

❗ Current head 0a0f8e2 differs from pull request most recent head 1e30c30. Consider uploading reports for the commit 1e30c30 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1667      +/-   ##
==========================================
- Coverage   73.03%   73.03%   -0.01%     
==========================================
  Files         228      228              
  Lines       13378    13389      +11     
==========================================
+ Hits         9771     9779       +8     
- Misses       2892     2894       +2     
- Partials      715      716       +1     

Impacted Files	Coverage Δ
x/ecocredit/marketplace/keeper/utils.go	49.66% <86.20%> (+6.80%)	⬆️
x/ecocredit/marketplace/keeper/msg_buy_direct.go	78.12% <94.11%> (+1.98%)	⬆️
app/app.go	92.39% <0.00%> (-0.29%)	⬇️

[blushi]

pre-approving, just a couple nits