Clear stale input states when triggers are removed and fix virtual LT/RT buttons in sdl backend

[riccardobl]

This PR solves two input related issues:

1. when all triggers for a mapping are removed, the state of the button/axis is cleared: before it was possible to remove all triggers while a button was pressed, leaving it permanently pressed.
2. Skip polling of virtual LT/RT buttons in SDL backend: for compatibility reasons the SDL backend creates two "virtual buttons" for RT/LT axis, but since they are not real button seen by SDL, the normal polling was overwriting the values computed by the backend.

[gemini-code-assist]

Code Review

This pull request modifies InputManager to clean up bindings, pressed buttons, and axis values when mappings or triggers are deleted, and adds corresponding unit tests to verify that deleted mappings/triggers clear the pressed key state. Additionally, it updates SdlJoystickInput to ignore Xbox LT and RT buttons during button event handling. The reviewer feedback highlights potential NullPointerException risks in both deleteTrigger and deleteMapping if the retrieved mapping list is null, suggesting defensive null checks to prevent crashes.

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[github-actions]

🖼️ Screenshot tests have failed.

The purpose of these tests is to ensure that changes introduced in this PR don't break visual features. They are visual unit tests.

📄 Where to find the report:

• Direct Download: screenshot-test-report
• Alternatively, go to the failed run summary > Artifacts > screenshot-test-report
• Download the zip and open jme3-screenshot-tests/build/reports/ScreenshotDiffReport.html

⚠️ If you didn't expect to change anything visual:
Fix your changes so the screenshot tests pass.

✅ If you did mean to change things:
Review the replacement images in jme3-screenshot-tests/build/changed-images to make sure they really are improvements and then replace and commit the replacement images at jme3-screenshot-tests/src/test/resources.

✨ If you are creating entirely new tests:
Find the new images in jme3-screenshot-tests/build/changed-images and commit the new images at jme3-screenshot-tests/src/test/resources.

Note; it is very important that the committed reference images are created on the build pipeline, locally created images are not reliable. Similarly tests will fail locally but you can look at the report to check they are "visually similar".

See https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-screenshot-tests/README.md for more information

Contact @richardTingle (aka richtea) for guidance if required