Add comprehensive tests for CopyToTemporaryStreamAsync overloads and fix plugin bugs

[Copilot]

This PR implements comprehensive automated tests for the two CopyToTemporaryStreamAsync extension method overloads as requested in the issue. The tests follow Sociable Unit Testing principles, testing real components in combination without test doubles.

🧪 Tests Added

8 comprehensive tests covering both overloads:

Basic CopyToTemporaryStreamAsync overload:

• ✅ MemoryStream scenario with small data (< 80KB)
• ✅ FileStream scenario with large data (> 80KB)
• ✅ Custom buffer size parameter
• ✅ Custom options parameter

Plugin-enabled CopyToTemporaryStreamAsync overload:

• ✅ HashingPlugin with SHA-1 algorithm (MemoryStream scenario)
• ✅ HashingPlugin with SHA-1 algorithm (FileStream scenario)
• ✅ Multiple hash algorithms (SHA-1 + SHA-256)
• ✅ Custom buffer size with plugins

All tests verify:

• Correct stream type selection (Memory vs File based on 80KB threshold)
• Accurate data copying and content verification
• Hash calculation correctness using SHA-1 as specified
• Parameter handling (buffer sizes, options, file paths)

🐛 Critical Bugs Fixed

During test development, two critical bugs were discovered and fixed in the existing codebase:

1. Plugin Setup Bug in TemporaryStreamServiceExtensions

Issue: Line 126 was incorrectly passing the source stream to plugin setup instead of the outermostStream (destination).

// Before (incorrect)
outermostStream = await plugins[i].SetUpAsync(source, cancellationToken).ConfigureAwait(false);

// After (correct)  
outermostStream = await plugins[i].SetUpAsync(outermostStream, cancellationToken).ConfigureAwait(false);

This caused plugins to wrap the wrong stream, leading to NotSupportedException: Memory stream is not expandable.

2. Multiple Hash Calculator Bug in HashingPlugin

Issue: The plugin wasn't properly chaining multiple hash calculators - all calculators were wrapping the same inner stream instead of creating a chain.

// Before (incorrect)
for (var i = 0; i < HashCalculators.Length; i++)
{
    outermostStream = HashCalculators[i].CreateWrappingCryptoStream(innerStream, leaveWrappedStreamOpen: true);
}

// After (correct)
Stream currentStream = innerStream;
for (var i = 0; i < HashCalculators.Length; i++)
{
    currentStream = HashCalculators[i].CreateWrappingCryptoStream(currentStream, leaveWrappedStreamOpen: true);
}

📁 Test Resources

Added sample PDF files as requested:

• less-than-80KB.pdf (596 bytes) - triggers MemoryStream usage
• more-than-80KB.pdf (80,602 bytes) - triggers FileStream usage

✅ Results

• All 103 tests pass (95 existing + 8 new)
• No breaking changes to existing functionality
• Comprehensive coverage of both extension method overloads
• Real-world demonstration with actual file size thresholds

Fixes #6.

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[feO2x]

LGTM - I fixed some things, but overall great work