Fix reading Zip64 end of central directory locator

[rzikm]

Fixes #117147.
Replaces #118230.

The structure of Zip64 ZIP file is as follows:

• local file header
• ....
• central directory
• ZIP64 end of central directory record
• ZIP64 end of central directory locator
• End of central directory record

The applications reading the ZIP archive are expected to parse it from the end, where presence of ZIP64 fields is indicated by special values in the eocd record.

the problematic input is too short to contain eocd locator, which triggers the assert, this PR adds a stream length check for that.

This PR also fixes some internal methods (SeekBackwardsToSignature(Async)) to now behave as expected. Previous implementation always read at least 4kb even if maxBytesToRead was less. This in turn uncovered a different bug where expected offset of eocd locator was not calculated correctly.

Review of the fuzzing code (ZipArchiveFuzzer) revealed some issues (like disposing the stream between runs and ignoring all exceptions), so this PR fixes improves the fuzzer az well.

[Copilot]

Pull Request Overview

This PR fixes a bug in reading Zip64 end of central directory locator by adding proper stream length validation and correcting internal signature seeking methods. The changes address issue #117147 where malformed ZIP archives with insufficient data could trigger assertions.

Key changes:

• Adds stream length check before attempting to read Zip64 end of central directory locator
• Fixes SeekBackwardsToSignature methods to respect maxBytesToRead parameter properly
• Corrects calculation of total bytes read and overlap handling in signature seeking

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
ZipHelper.cs	Fixed SeekBackwardsToSignature to properly respect maxBytesToRead and calculate overlap correctly
ZipHelper.Async.cs	Applied same fixes to async version of SeekBackwardsToSignature method
ZipArchive.cs	Added stream length validation and simplified EOCD position tracking
ZipArchive.Async.cs	Applied same validation and position tracking fixes to async version

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[rzikm]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[MihaZupan]

@MihuBot fuzz ZipArchive

[rzikm]

I think we are ready for review.