refactor(linux): Extract GTK3 display monitor backend in preparation for GTK4 - #195324

Summary

Part of #194698, which tracks incremental refactors of the GTK3 Linux embedder to enable GTK4 support cleanly.

This PR focuses on display monitoring. It moves GTK3-specific monitor discovery, scale-factor lookup, and hotplug signal subscriptions behind a private backend, while keeping display ID management and engine notifications in the shared FlDisplayMonitor implementation.

This separation provides a clear integration point for a future GTK4 backend and helps keep toolkit-specific conditionals out of the shared display-management code. It is one independently reviewable piece of the preparatory work supporting #94804.

It also fixes duplicate signal subscriptions when fl_display_monitor_start() is called more than once. Each call still publishes the current display snapshot, while subsequent monitor additions and removals produce a single update.

Changes

• Introduce a private FlDisplayMonitorBackend interface, with the GTK3 implementation in fl_display_monitor_gtk3.cc.
• Move monitor enumeration, scale-factor lookup, and monitor-added / monitor-removed subscriptions into the backend.
• Return owned snapshots containing referenced monitor objects, and disconnect signal handlers when the backend is freed.
• Keep display ID assignment, display metadata construction, and engine notifications in FlDisplayMonitor.
• Preserve existing logical dimensions, scale factors, refresh-rate conversion, and display ID behavior: surviving monitors retain their IDs, and removed monitors receive new IDs if added again.
• Ensure repeated startup subscribes to monitor signals only once while continuing to refresh the display snapshot.

Testing

Extend the GTK mocks to support configurable monitor lists, geometry, scale factors, and refresh rates. Add seven regression tests covering:

• Multiple monitors and the display metadata reported to the engine.
• Monitor removal and readdition, including preservation of surviving display IDs.
• Empty monitor lists and removal of the final monitor.
• Stable display IDs when monitor enumeration order changes.
• Repeated startup refreshing the snapshot without duplicating hotplug notifications.
• Disconnection of both monitor signals on disposal.
• Safe handling of monitor notifications after engine destruction.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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