fix: add missing charset encoding for UTF8MB4_0900_BIN

[kangsorang]

Fix charset encoding mapping for UTF8MB4_0900_BIN

Problem

MySQL2 throws Encoding not recognized: 'undefined' error when connecting to MySQL 8.0+ servers using utf8mb4_0900_bin collation.

Root Cause

Mismatch between charsets.js and charset_encodings.js:

• charsets.js defines UTF8MB4_0900_BIN = 309
• charset_encodings.js array has length 309 (indices 0-308)
• charset_encodings[309] returns undefined
• Iconv.getDecoder(undefined) throws encoding error

Solution

Add missing encoding mapping for index 309 in charset_encodings.js.

Changes

• File: lib/constants/charset_encodings.js
• Change: Add 'utf8' entry at index 309 for UTF8MB4_0900_BIN

Impact

• Fixes connection issues with MySQL 8.0+ utf8mb4_0900_bin collation
• Resolves Encoding not recognized: 'undefined' errors
• Fix : Encoding not recognized: 'undefined' error when connecting to MySQL 8.0+ servers that use utf8mb4_0900_bin collation. #3856

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (8143e42) to head (cb79669).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3855      +/-   ##
==========================================
- Coverage   89.78%   89.76%   -0.02%     
==========================================
  Files          86       86              
  Lines       13605    13606       +1     
  Branches     1606     1606              
==========================================
- Hits        12215    12214       -1     
- Misses       1390     1392       +2     

Flag	Coverage Δ
compression-0	88.65% <100.00%> (-0.25%)	⬇️
compression-1	86.68% <100.00%> (-3.08%)	⬇️
static-parser-0	87.35% <100.00%> (+<0.01%)	⬆️
static-parser-1	86.93% <100.00%> (-1.19%)	⬇️
tls-0	89.11% <100.00%> (-0.09%)	⬇️
tls-1	88.65% <100.00%> (-0.91%)	⬇️

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[wellwelwel]

Thanks, @kangsorang ✨

Do you think it's possible to have a test that covers this case (reproducing the failure if it doesn't have your modification)? Perhaps a temporary table with this encoding would solve it.