RDP/rdpsnd: add Wave2 audio compatibility for modern FreeRDP/GRD servers

[chrilep]

Summary

This PR adds compatibility for SNDC_WAVE2 in the RDP rdpsnd channel and updates negotiated rdpsnd version handling so audio works with modern server implementations (including GNOME Remote Desktop / FreeRDP-based paths) that use Wave2 semantics.

Problem

In affected environments, RDP audio negotiation succeeds but audio payloads arrive in SNDC_WAVE2 format while Guacamole only handled classic SNDC_WAVE. Result: no audio in Guacamole despite successful session setup.

Change

• Add SNDC_WAVE2 handling in rdpsnd message dispatch
• Implement Wave2 message handling alongside existing Wave flow
• Update rdpsnd version negotiation handling to interoperate with Wave2-capable servers

Impact

• Restores/introduces audio playback in environments that emit Wave2 audio PDUs
• Keeps existing Wave behavior intact for older servers

Scope

Targeted protocol compatibility update in rdpsnd path only.

AI Assistance Disclosure

Parts of this contribution (analysis and code drafting) were created with assistance from GitHub Copilot (GPT-5.3-Codex) and then reviewed/refined before submission.

[necouchman]

@chrilep Thank you for the contribution to the project. As a starting point, you will need to submit a Jira issue for these changes (and apply for a Jira account, if you don't already have one) and then prefix both the PR title and the commit messages with the Jira issue.

https://issues.apache.org/jira/browse/GUACAMOLE

You can see full contribution guidelines, here: https://guacamole.apache.org/open-source/

And you can look at other open pull requests to see examples of how that looks once you have your Jira issue.

[chrilep]

Aye, I registered at Jira.