[material_ui] Ensure textScaler is taken into account for SnackBar action overflow

[matasb-google]

Port of flutter/flutter#181959 to flutter/packages as part of the Material decoupling effort (flutter/flutter#188444).

Ensure textScalling is taken into account for Sizing overflow.

Fixes flutter/flutter#192232
Fixes flutter/flutter#181959
Part of flutter/flutter#188444

Description

• Ensure textScalling is taken into account for Sizing overflow: passes MediaQuery.textScalerOf(context) to actionTextPainter so that actionAndIconWidth properly scales with accessibility text scaling.
• Removes SizedBox(width: snackBarWidth * 0.4) when the action overflows, eliminating unintended blank space on the right of multi-line content.
• Adds unit tests in snack_bar_test.dart verifying textScaler overflow calculation and layout.
• Adds pending changelog entry.

Pre-Review Checklist

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• 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.
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• 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.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request updates the SnackBar implementation to respect the MediaQuery's TextScaler when calculating action overflow, and removes the unnecessary 40% blank right spacing in multi-line layouts. It also adds corresponding widget tests to verify these behaviors. There are no review comments, so I have no feedback to provide.

[matasb-google]

@Piinks This has now been ported over from flutter/flutter#181959 with unit tests and pending changelog following #188444. Ready for review!