Explicitly handle missing SOS marker.

[JimBobSquarePants]

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Description

Fix #2948

[Copilot]

Pull Request Overview

This PR adds explicit validation to detect and handle JPEG images that are missing the required SOS (Start of Scan) marker, fixing issue #2948. Previously, such malformed images could cause unexpected behavior; now they will throw a clear InvalidImageContentException.

• Added a boolean flag to track whether the SOS marker is encountered during JPEG parsing
• Added validation after stream parsing in both decode and identify code paths to throw an exception if SOS marker is missing
• Added comprehensive test coverage for both decode and identify operations with a malformed test image

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/Images/Input/Jpg/issues/issue-2948-sos.jpg	Test image file (stored in Git LFS) that is missing the SOS marker
tests/ImageSharp.Tests/TestImages.cs	Added constant reference to the new test image
tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs	Added two test methods to verify that both decode and identify operations throw InvalidImageContentException for images missing SOS marker
src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs	Added hasSOSMarker field, set it when SOS marker is encountered, and added validation checks after ParseStream in both Decode and Identify methods

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