[material_ui] Migrate M3 Motion template to use new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899.
Fixes flutter/flutter#188422.

This PR migrates the M3 Motion defaults template to use the new gen_defaults in material_ui.
The generated defaults remain unchanged.

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[gemini-code-assist]

Code Review

This pull request migrates the Material 3 motion defaults to a template-based generator (MotionTemplateM3), moving the hardcoded tokens from motion.dart into a generated file and introducing a helper Cubic struct. The generation tool and tests are updated to support this template. The review feedback suggests improving the formatting of the generated code templates in motion_template.dart by using .trimRight() to place the closing braces of the generated classes on separate lines.

[gemini-code-assist]

The closing brace } is placed directly after the string interpolation as }}, which can be hard to read and error-prone during future edits. Using .trimRight() on the last token string allows placing the closing brace on its own line without introducing an extra blank line.

Suggested change

	${_durationTokenString('extralong4', TokenMotionStandard.durationExtraLong4)}}
	${_durationTokenString('extralong4', TokenMotionStandard.durationExtraLong4).trimRight()}
	}

[gemini-code-assist]

The closing brace } is placed directly after the string interpolation as }}, which can be hard to read and error-prone during future edits. Using .trimRight() on the last token string allows placing the closing brace on its own line without introducing an extra blank line.

${_easingCurveTokenString('legacy', TokenMotionStandard.easingLegacy).trimRight()}
}

[elliette]

LGTM but we should use the new CubicStruct added in #13082 instead