feat(m17): enhance parrot mode and fix @ALL routing

[dbehnke]

M17 Protocol Enhancements and Fixes

Overview

This PR implements significant improvements and fixes for the M17 protocol handling in urfd.

Key Changes

1. @ALL Receive Fix ("Contextual Broadcast")

• Issue: M17 @ALL packets were being rejected due to strict callsign validation ('@' not allowed) and empty module information from mvoice.
• Fix:
  • Updated CCallsign::IsValidM17 to allow @ and # prefixes.
  • Implemented logic in M17Protocol to detect @ALL with an empty module and force the target module to the client's current module.
  • Fixed string comparison to ignore trailing spaces when detecting @ALL.

2. Parrot Mode Implementation

• Feature: Full support for M17 Parrot (Echo) mode.
• Trigger: Accepts destination callsigns containing "PARROT" (e.g., PARROT, M17-PARROT, #PARROT), matching mrefd fuzzy matching behavior.
• Safeguards:
  • Recording Limit: Hardcoded 30-second limit (750 frames) to prevent memory abuse.
  • Playback Delay: Added a 5-second delay (timeout) before playback starts to allow network settling.
• Fixes: Resolved a variable shadowing bug isParrot that caused module lookup errors.

3. Stream Timeout Fix

• Issue: Regular M17 streams (non-parrot) were getting stuck open because the protocol loop wasn't checking for stream expiration.
• Fix: Added call to base CProtocol::CheckStreamsTimeout() ensuring idle streams are closed properly.

4. Packet Mode (SMS)

• Improved routing for M17 Packet Mode (SMS) data to ensure it follows the same validation rules.

Verification

• Verified compilation and build via Docker.
• Tested against mvoice client (verified Parrot functionality and error handling).