fix(pgwire): fix the wrong typeOID of Decimal type

[ZENOTME]

What's changed and what's your intention?

• fix the wrong typeOID of Decimal type.
• Add the comment to avoid bug like this in the future.

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

#3015

[TennyZhuang]

LGTM

[BowenXiao1999]

cc @xiangjinwu, PTAL

[xiangjinwu]

lgtm

[codecov]

Codecov Report

Merging #3280 (bd1a349) into main (53a49d8) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3280      +/-   ##
==========================================
- Coverage   73.23%   73.23%   -0.01%     
==========================================
  Files         747      747              
  Lines      101789   101789              
==========================================
- Hits        74545    74543       -2     
- Misses      27244    27246       +2     

Flag	Coverage Δ
rust	73.23% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
src/utils/pgwire/src/pg_field_descriptor.rs	76.92% <0.00%> (ø)
src/meta/src/hummock/mock_hummock_meta_client.rs	41.48% <0.00%> (-1.07%)	⬇️
src/storage/src/hummock/local_version_manager.rs	83.86% <0.00%> (-0.16%)	⬇️

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