feat(BaseConnection): add state getter to track connection lifecycle

[CHOIJEWON]

Summary

Closes #3764

Adds a state getter to Connection for tracking connection lifecycle states

Motivation

Users need a reliable way to monitor connection status for debugging and health checks. Previously, there was no standard property
to check the current connection state.

Changes

• New property: connection.state (readonly)
• States: disconnected → protocol_handshake → connected → authenticated / error
• TypeScript: Added ConnectionState type definition
• Tests: 11 unit tests covering all states and priority rules

Usage

const connection = mysql.createConnection(config);

console.log(connection.state); // 'disconnected'

connection.connect((err) => {
  if (err) {
    console.log(connection.state); // 'error'
  } else {
    console.log(connection.state); // 'authenticated'
  }
});

State Priority

Error > Disconnected > Authenticated > Connected > Protocol Handshake

Test Results

✔ All 11 state transition tests passing

[codecov]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.78%. Comparing base (9049277) to head (5cbbac8).

Files with missing lines	Patch %	Lines
lib/base/connection.js	90.32%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3958   +/-   ##
=======================================
  Coverage   89.78%   89.78%           
=======================================
  Files          86       86           
  Lines       13607    13638   +31     
  Branches     1607     1616    +9     
=======================================
+ Hits        12217    12245   +28     
- Misses       1390     1393    +3     

Flag	Coverage Δ
compression-0	88.89% <90.32%> (+<0.01%)	⬆️
compression-1	89.76% <90.32%> (+<0.01%)	⬆️
static-parser-0	87.36% <90.32%> (+<0.01%)	⬆️
static-parser-1	88.12% <90.32%> (+<0.01%)	⬆️
tls-0	89.19% <90.32%> (+<0.01%)	⬆️
tls-1	89.55% <90.32%> (+<0.01%)	⬆️

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