(0.15.x backport) Hostname creation cleanup, avoid that default DNS name ESP-xxxxx gets used

[softhack007]

Bug Fixes

• Improved hostname sanitization and buffer-size safeguards to prevent truncation and ensure DNS-compliant names.
• Hostname is now applied earlier during network setup and on reconnects (ESP32), improving connectivity and ensuring settings take effect.
• Solves the problem that ESP32 boards show as "ESP-xxxxxx" on the network, no matter if DNS name is defined or not

New Features

• Added a length-aware hostname retrieval API with an mDNS-preference option and robust fallback behavior.
• Preserved legacy compatibility by providing a length-aware alias for legacy hostname preparation.

Limitations

• mDNS name is not sanitized when registering the mDNS endpoint, to avoid breaking previous functionality
• hostnames with double WLED- prefix, like wled-WLED-kitchen still allowed, to prevent breaking legacy behaviour

Summary by CodeRabbit

• Bug Fixes
  • Enhanced hostname validation and generation to improve reliability.
  • Improved network initialization with safer hostname assignment across WiFi and Ethernet interfaces.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch backport_DNS-fix

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