Consolidate ArrayPool + ReadFully/ReadExact patterns into reusable utilities

[Copilot]

Multiple files contained identical ArrayPool rent/return/try-finally patterns around stream reads. This consolidates them into helper methods.

Changes

Added utilities in Utility.cs and Utility.Async.cs:

• WithRentedBufferReadFully<T>() - Rents buffer, reads until full, executes callback, returns buffer
• TryWithRentedBufferReadFully<T>() - Same but returns false on EOF instead of throwing
• WithRentedBufferReadExact<T>() - Throws on partial read

Replaced duplicate patterns in:

• ArcFactory.cs - IsArchive signature check
• SevenZipArchive.Factory.cs - Signature validation
• GZipArchive.Factory.cs - Header validation
• WinzipAesCryptoStream.cs - Auth bytes disposal

Before/After

// Before: Manual ArrayPool management (repeated ~6 times)
var buffer = ArrayPool<byte>.Shared.Rent(2);
try
{
    if (stream.ReadFully(buffer.AsSpan(0, 2)))
    {
        return buffer[0] == 0x1A && buffer[1] < 10;
    }
    return false;
}
finally
{
    ArrayPool<byte>.Shared.Return(buffer);
}

// After: Consolidated helper
return stream.TryWithRentedBufferReadFully(
    2,
    buffer => buffer[0] == 0x1A && buffer[1] < 10,
    out var result
) && result;

Removes ~85 lines of boilerplate. All 1160 existing tests pass.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Consolidate ReadFully/ReadExact and other instances of things</issue_title>
<issue_description>Inspired by #1169

Surely, this same logic can be reused in places.</issue_description>

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

• Fixes Consolidate ReadFully/ReadExact and other instances of things #1170

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