[material_ui] Fix Switch throwing when first painted mid-animation - #13496

_SwitchPainter only initializes its pressed-state fields inside paint(). If the painter's first paint happens in an unusual animation state, those fields are read before they are ever written:

This can happen, for example, when a Switch.adaptive in a ListView's cache extent (laid out but not painted) changes value and is scrolled into view during its toggle animation, or when a switch under Opacity(0) is long-pressed and becomes visible in onChanged.

This PR defaults _pressedThumbExtension to 0 and falls back to the unpressed thumb radii in the Cupertino branch, as the Material branch already does. Initializing _pressedThumbExtension alone (as suggested in the issue) is not enough, because the iOS/macOS null check error comes from _pressedInactiveThumbRadius!.

Two regression tests cover both triggers on all platforms (TargetPlatformVariant.all()). Without the fix, 8 of the 12 variants fail.

Fixes flutter/flutter#179532

%23%23 Pre-Review 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 page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the %23hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.