Clear the window on teardown so Start can run again

[matt-edmondson]

Fixes #404

What was wrong

Start()'s teardown disposed the window but never cleared the static window field. The guard at the top of Start() reads that field, so every later call threw InvalidOperationException("Application is already running.") for the rest of the process — with nothing actually running. Reset() clears the field, but it is internal and visible only to the test assembly, so consumers had no workaround. Restart-to-apply-settings, and any harness driving sequential sessions in one process, were impossible.

EmbeddedSession.RunLoop's finally block already cleared the field, with a comment saying it does so by design, so the two lifecycle paths had drifted.

The change

Both paths now end in one TeardownWindow(), rather than each carrying its own list of fields to clear:

• disposes the controller, the input context and the GL context (via the existing Cleanup* helpers, all of which are idempotent — the window's Closing handler has usually run them already), then disposes the window and clears it
• Start()'s blocking branch is factored into RunWindowLoop(), which runs the loop and then tears down; this also makes the post-loop path reachable from a test without a real window
• EmbeddedSession.RunLoop's finally calls the same helper, so its teardown now also releases the GL resources on an abnormal exit

The embedded path gains GL/controller/input cleanup it did not have; nothing else changes behaviour.

Tests

New tests/ImGui.App.Tests/WindowTeardownTests.cs:

• RunWindowLoopDisposesTheWindowAndClearsIt — the window is disposed and the field is null once the loop returns
• RunWindowLoopReleasesTheResourcesThatHungOffTheWindow — the input context is disposed, and controller, renderer, gl and glProvider are cleared
• StartIsAllowedAgainOnceTheRunLoopHasReturned — the issue's acceptance criterion: Start → loop returns → Start again succeeds

All three fail against the old teardown (the third with the exact Application is already running. from the issue) and pass with the fix. Full ImGui.App.Tests suite: 431/431 passing on this branch.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AZcLBdphhSBSNeUALeVhxB

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
84.6% Coverage on New Code
0.0% Duplication on New Code

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