[Animations] Add support for custom shadows to OpenContainer (#62475)

[moepanda]

Summary

This PR adds support for custom BoxShadow to the OpenContainer widget. This allows developers to specify closedShadows and openShadows to create more advanced and visually appealing transitions that go beyond the standard material elevation.

• Fixes a legacy layout overflow issue in _ExampleSingleTile by replacing the fixed height with responsive constraints, ensuring the example remains stable across different system font sizes and shadow distributions.

Motivation

Currently, OpenContainer only supports a numeric elevation property, which limits modern UI designs requiring colored or multi-layered shadows. This PR also refactors the internal rendering hierarchy to move the shadow decoration outside the clipping surface, fixing a legacy issue where custom shadows were clipped by Material.

Issues

Fixes flutter/flutter#62475

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.
• 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]. (Reminder: Please ensure you've signed it!)
• The title of the PR starts with [animations]
• I linked to at least one issue that this PR fixes in the description above.
• I followed [the version and CHANGELOG instructions].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request adds support for custom closedShadows and openShadows to the OpenContainer widget, enabling custom shadow animations during transitions. It also fixes a layout overflow in the example app by transitioning from a fixed height to minimum height constraints and includes new tests to verify shadow behavior. A review comment suggests conditionally wrapping the Material widget with DecoratedBox only when custom shadows are provided to avoid unnecessary widgets in the tree.

[gemini-code-assist]

Wrapping the Material widget with a DecoratedBox even when closedShadows is null adds an unnecessary widget to the tree. While the performance impact is likely negligible, you could conditionally wrap it to keep the tree cleaner in the default case.

        child: widget.closedShadows == null
            ? Material(
                clipBehavior: widget.clipBehavior,
                color: widget.closedColor,
                elevation: widget.closedElevation,
                shape: widget.closedShape,
                child: Builder(
                  key: _closedBuilderKey,
                  builder: (BuildContext context) {
                    return widget.closedBuilder(context, openContainer);
                  },
                ),
              )
            : DecoratedBox(
                decoration: ShapeDecoration(
                  shape: widget.closedShape,
                  shadows: widget.closedShadows,
                ),
                child: Material(
                  clipBehavior: widget.clipBehavior,
                  color: widget.closedColor,
                  elevation: 0.0,
                  shape: widget.closedShape,
                  child: Builder(
                    key: _closedBuilderKey,
                    builder: (BuildContext context) {
                      return widget.closedBuilder(context, openContainer);
                    },
                  ),
                ),
              ),