Description

Container's clipBehavior property was not clipping the decoration painting because ClipPath was placed as a child of DecoratedBox in the widget tree. This meant only the child content was clipped, while the decoration itself (background color, gradient, border, shadow) was painted outside the clip boundary.

This caused visible artifacts when a Container with a rounded BoxDecoration and clipBehavior set was placed inside a parent with a solid background color: the parent's color would show through at the corners of the rounded decoration, and clipBehavior had no effect on the decoration's visual output.

Before (broken):

DecoratedBox        <-- paints decoration OUTSIDE clip
  └── ClipPath      <-- only clips child content
       └── child

After (fixed):

ClipPath            <-- clips everything: decoration + child
  └── DecoratedBox  <-- decoration painting is now clipped
       └── child

This fix swaps the order of ClipPath and DecoratedBox in Container.build() so that ClipPath wraps DecoratedBox, ensuring the clip applies to the entire visual output of the container (decoration + child content).

Shadow behavior change

Previously, boxShadow was not clipped because DecoratedBox painted it outside the ClipPath. With this fix, shadows are now also clipped to the decoration shape when clipBehavior is set. This is the correct behavior because:

• clipBehavior defaults to Clip.none, so shadows are unaffected unless the developer explicitly opts into clipping
• Developers who set clipBehavior expect the container's visual bounds to be constrained to the decoration shape
• This is consistent with how ClipRRect and ClipPath work when wrapping decorated content

The shadow golden test has been updated to reflect this corrected behavior.

Fixes #85268

Pre-launch 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 wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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