Revert auth fixes, keep only Protocol 16/17 and new data types

Removed authentication-related fixes to make minimal changes:
- Reverted sendOpContAuth to original (no Buffer conversion)
- Reverted op_cont_auth handler (no _pendingAccept logic)
- Reverted op_cond_accept keys reading (back to readString)
- Removed all debug logging statements
- Removed getOpcodeName helper function
- Removed auth fix documentation files

Kept only Protocol 16/17 features:
- Protocol 16 and 17 constants (PROTOCOL_VERSION16/17)
- DECFLOAT data type support (SQL_DEC16, SQL_DEC34, blr_dec64, blr_dec128)
- DECFLOAT encoding/decoding methods in serialize.js
- DECFLOAT SQL variable classes in xsqlvar.js
- INT128 type constants
- Extended metadata identifier support
- Protocol tests for new features

This makes the changes minimal and focused only on new protocol
features and data types introduced in Firebird 4.0.

Co-authored-by: mariuz <18359+mariuz@users.noreply.github.com>
Agent-Logs-Url: https://github.com/hgourvest/node-firebird/sessions/b56c048d-c258-45a2-b823-66d9c0f8c98e

Author: Copilot