Fix Image dimension DefaultValue metadata

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Fixes dotnet/runtime#126265

Proposed changes

• Change the DefaultValue metadata for Image.Width and Image.Height from false to integer 0.
• Add a parameterized TypeDescriptor regression test for both properties.

Customer Impact

• Metadata consumers no longer receive a Boolean default for integer properties.
• Prevents invalid-cast failures in consumers such as OpenAPI schema generation.

Regression?

• No. This corrects invalid design-time metadata; the property values and API surface are unchanged.

Risk

• Low. The production change is limited to two DefaultValue attributes and is covered by a focused regression test.

Test methodology

• Built System.Drawing.Common and System.Drawing.Common.Tests with the repository-pinned .NET 11 toolchain.
• Ran System.Drawing.Tests.ImageTests.ImageDimensionProperty_DefaultValueIsZero: 2 passed, 0 failed.

Test environment(s)

• Windows x64
• .NET SDK 11.0.100-preview.6.26359.118
• Test runtime 11.0.0-rc.1.26411.119

AI assistance: Codex was used to prepare this change; the patch was reviewed by the submitter.

Microsoft Reviewers: Open in CodeFlow

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