fix: Assorted Oracle fixes

[mvadari]

High Level Overview of Change

This PR:

• Switches a potential negative variable from unsigned to signed
• Fixes some inaccurate comments
• Improves performance of get_aggregate_price by preventing unnecessary repeat computation

Context of Change

AI code review

API Impact

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.4%. Comparing base (cf2eb14) to head (92cab2e).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6570   +/-   ##
=======================================
  Coverage     81.4%   81.4%           
=======================================
  Files          988     988           
  Lines        74411   74411           
  Branches      7560    7557    -3     
=======================================
+ Hits         60599   60605    +6     
+ Misses       13812   13806    -6     

Files with missing lines	Coverage Δ
src/libxrpl/tx/transactors/oracle/OracleSet.cpp	98.8% <100.0%> (ø)
src/xrpld/rpc/handlers/GetAggregatePrice.cpp	99.4% <100.0%> (ø)

... and 2 files with indirect coverage changes

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