[material_ui] Keep ExpansionPanelList dividers when expanded with 0 materialGapSize (#183974)

[benji-farquhar]

Fixes flutter/flutter#183974
Supersedes flutter/flutter#183975

Ports the change originally submitted in flutter/flutter#183975 to packages/material_ui.

cc @dkwingsmt @victorsanni who previously reviewed and approved the original PR in flutter/flutter#183975.

Description

When ExpansionPanelList.materialGapSize is set to 0, MaterialGap is no longer inserted between expanded panels. This allows MergeableMaterial to preserve the dividers between adjacent expanded panels, creating a clean flat list appearance (ideal for nested lists or panels within cards).

This is the end result of the PR (before it, the dividers between panels were absent):

• Guarded MaterialGap insertion in ExpansionPanelList with materialGapSize > 0.
• Documented zero-gap behavior in materialGapSize with a <callout-box>.
• Added example sample (expansion_panel_list.2.dart) and widget tests.
• Updated expansion_panel_test.dart to verify gap insertion and zero-gap divider retention.
• Added pending changelog entry.

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.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request modifies ExpansionPanelList to preserve dividers and avoid inserting a MaterialGap when materialGapSize is set to 0. It includes documentation updates, a new example, and corresponding tests. The review feedback identifies syntax errors in the new example file where the EdgeInsets prefix was omitted on several padding properties.