Avoid exceeding allocated buffers when doing MQTT proxying

[dirkmueller]

the topic length could be longer than 65 characters. similarly for the payload.

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
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[Copilot]

Pull Request Overview

This PR fixes potential buffer overflow vulnerabilities in MQTT proxying by replacing unsafe string copying with bounds-checked operations and adding payload size validation.

• Replaces strcpy with strlcpy to prevent topic buffer overflow
• Adds length validation to prevent payload buffer overflow
• Ensures safe copying of MQTT data within allocated buffer boundaries

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