fix(display): validate Wayland socket paths

[unbridled-41]

Summary

Reject empty Wayland display names and filesystem paths that do not refer to Unix sockets.

Root cause

getWaylandDisplay() indexed display[0] without checking for an empty string. It also treated any existing path as a valid display socket, so regular files and directories could be returned to callers.

Changes

• Reject an empty WAYLAND_DISPLAY value before indexing it.
• Inspect the resolved path with std::filesystem::status().
• Require the path type to be a Unix socket.
• Add tests for empty input, regular files, directories, and real absolute/relative sockets.

Test plan

• Release build with the complete fix set
• Full CTest suite: 528 passed, 2 unrelated environment-dependent X11 tests skipped, 0 failed
• Added four focused Wayland validation cases
• git diff --check

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: unbridled-41

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[deepin-ci-robot]

Hi @unbridled-41. Thanks for your PR.

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