Add headless client CI builds and Docker packaging

[Toys0125]

Summary

This PR adds a dedicated headless client build and packaging flow for Linux and Windows, plus the runtime changes needed to make those builds usable in Docker-based deployments.

What changed

• Added GitHub Actions jobs to build headless Unity players for Linux and Windows
• Added Docker packaging for both headless outputs
• Added CI sanitization to strip XR/OpenVR/sample content that was breaking headless builds
• Added a custom Unity headless build entrypoint for server-target builds
• Updated headless runtime config loading to read environment variables first, then fall back to config.xml
• Fixed headless input scaling and disconnect handling issues discovered during validation
• Disabled HDR/URP settings application on server builds
• Switched the server scripting backend to Mono for dedicated-server compatibility
• Normalized published headless artifact folder names to HeadlessLinux64 and HeadlessWindows64

Why

The goal of this branch is to make headless client/server builds reproducible in CI and usable in containerized deployments, while still allowing traditional file-based configuration.

In particular:

• Docker deployments need env-var-driven configuration
• Headless CI needed project sanitization to avoid XR/OpenVR and sample-content build failures
• Dedicated server packaging needed stable output folder names and Dockerfiles for both target OSes

Validation

• Latest GitHub Actions run completed successfully
• Headless Linux build succeeded
• Headless Windows build succeeded
• Docker packaging flow succeeded with the normalized artifact layout

Notes

• Headless runtime config now supports env vars first for Password, Ip, and Port, with config.xml as fallback
• The branch is intentionally squashed to a single commit for reviewability