Fix reachable Debug.Assert in InterleavedZipPackagePartStream.Length

[Copilot]

• Understand the issue: Debug.Assert(CanSeek) in InterleavedZipPackagePartStream.Length fails in debug builds
• Research behavior: In release builds, Length returned correct values because ZipWrappingStream.Length gets length from ZipArchiveEntry.Length even when CanSeek is false
• Fix: Remove the incorrect Debug.Assert(CanSeek) entirely - the underlying streams support Length even when not seekable
• Add test InterleavedZipPackagePartStream_Length_ReturnsCorrectValueWhenCanSeekIsFalse with exact length verification
• Run tests to validate changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Reachable Debug.Assert in InterleavedZipPackagePartStream.Length</issue_title>
<issue_description>In System.IO.Packaging, InterleavedZipPackagePartStream.Length asserts that CanSeek is true here:

runtime/src/libraries/System.IO.Packaging/src/System/IO/Packaging/InterleavedZipPackagePartStream.cs

Line 480 in d187228

Debug.Assert(CanSeek);

However this is not always true, and this assert is reachable from public APIs.

Using the existing PartPieceTests test fixture and data, the following unit test will hit the Debug.Assert:

[Fact]
public void InterleavedZipPackagePartStream_Length_Assert()
{
    using MemoryStream package = new(_partPieceSampleZipPackage);

    using Package zipPackage = Package.Open(package, FileMode.Open, FileAccess.Read);
    PackagePart partEntry = zipPackage.GetPart(new Uri("/ReadablePartPieceEntry.bin", UriKind.Relative));
    using Stream stream = partEntry.GetStream(FileMode.Open);
    Assert.NotEqual(0, stream.Length);
}

The assertion:

Stack Trace:
     at Microsoft.VisualStudio.TestPlatform.TestHost.TestHostTraceListener.GetException(String message)
   at Microsoft.VisualStudio.TestPlatform.TestHost.TestHostTraceListener.Fail(String message, String detailMessage)
   at System.Diagnostics.TraceInternal.Fail(String message, String detailMessage) in /Users/vcsjones/Projects/runtime/src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/TraceInternal.cs:line 261
   at System.Diagnostics.TraceInternal.TraceProvider.Fail(String message, String detailMessage) in /Users/vcsjones/Projects/runtime/src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/TraceInternal.cs:line 17
   at System.Diagnostics.Debug.Fail(String message, String detailMessage) in /Users/vcsjones/Projects/runtime/src/libraries/System.Private.CoreLib/src/System/Diagnostics/Debug.cs:line 135
   at System.Diagnostics.Debug.Assert(Boolean condition, String message, String detailMessage) in /Users/vcsjones/Projects/runtime/src/libraries/System.Private.CoreLib/src/System/Diagnostics/Debug.cs:line 99
   at System.Diagnostics.Debug.Assert(Boolean condition, String message) in /Users/vcsjones/Projects/runtime/src/libraries/System.Private.CoreLib/src/System/Diagnostics/Debug.cs:line 88
   at System.IO.Packaging.InterleavedZipPackagePartStream.get_Length() in /Users/vcsjones/Projects/runtime/src/libraries/System.IO.Packaging/src/System/IO/Packaging/InterleavedZipPackagePartStream.cs:line 480
   at System.IO.Packaging.Tests.PartPieceTests.InterleavedZipPackagePartStream_Length_Assert() in /Users/vcsjones/Projects/runtime/src/libraries/System.IO.Packaging/tests/PartPieceTests.cs:line 609

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Reachable Debug.Assert in InterleavedZipPackagePartStream.Length #123816

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[Copilot]

Pull request overview

This PR fixes a reachable Debug.Assert in InterleavedZipPackagePartStream.Length that fails in debug builds when accessing the Length property of a package part stream opened with read-only access.

Changes:

• Removed incorrect Debug.Assert(CanSeek) from the Length property getter in InterleavedZipPackagePartStream
• Added comprehensive test that verifies Length returns correct value even when CanSeek is false

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.IO.Packaging/src/System/IO/Packaging/InterleavedZipPackagePartStream.cs	Removed the incorrect assertion that assumed CanSeek must be true for Length to work
src/libraries/System.IO.Packaging/tests/PartPieceTests.cs	Added test that verifies Length works correctly when CanSeek is false, validating the exact byte count