fix(mail): avoid zip temp collisions

[PaulStSmith]

Summary

• Generate unique temporary zip file names for compressed mail attachments
• Prevent concurrent test/application runs from sharing the same second-level timestamp zip path
• Add regression coverage for repeated compression operations using distinct zip names

Tests

• dotnet test .\Toolkit.Modern.Tests\ByteForge.Toolkit.Modern.Tests.csproj --configuration Debug --no-restore --filter FullyQualifiedName~EmailAttachmentHandlerTests
• net9.0: 19 passed
• net48: 19 passed
• dotnet test .\Toolkit.Modern.Tests\ByteForge.Toolkit.Modern.Tests.csproj --configuration Debug --no-restore
• net9.0: 967 passed
• net48: 967 passed

[Copilot]

Pull request overview

This PR updates mail attachment compression to avoid temporary zip filename collisions by making generated zip paths unique, and adds regression coverage for repeated compression runs.

Changes:

• Uses UTC timestamp precision plus a GUID when generating compressed attachment temp zip names.
• Adds a regression test ensuring repeated compression produces distinct zip attachment names.

Reviewed changes

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

File	Description
Toolkit.Modern/Mail/EmailAttachmentHandler.cs	Updates compressed attachment temp zip filename generation to include higher precision time and a GUID.
Toolkit.Modern.Tests/Unit/Mail/EmailAttachmentHandlerTests.cs	Adds a unit test covering repeated compression operations with unique zip names.

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