Fix build warnings and clean up exception message

[YunchuWang]

• Remove duplicate BenchmarkDotNet 0.14.0 entry in Directory.Packages.props (fixes NU1506)
• Upgrade ExportHistoryWebApp from net6.0 to net8.0 (fixes NETSDK1138)
• Remove 'TODO' prefix from exception message in TaskHubGrpcServer

Summary

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Project checklist

• Release notes are not required for the next release
  • Otherwise: Notes added to release_notes.md
• Backport is not required
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• All required tests have been added/updated (unit tests, E2E tests)
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[Copilot]

Pull request overview

This PR addresses build warnings and improves code quality by removing a duplicate package reference, upgrading a sample to a supported .NET version, and cleaning up a user-facing exception message.

Changes:

• Removed duplicate BenchmarkDotNet 0.14.0 entry from Directory.Packages.props (fixes NU1506 warning)
• Upgraded ExportHistoryWebApp sample from net6.0 to net8.0 (fixes NETSDK1138 warning and aligns with other samples)
• Removed 'TODO' prefix from exception message in TaskHubGrpcServer and improved punctuation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Directory.Packages.props	Removed duplicate BenchmarkDotNet package version entry (0.14.0), keeping the newer 0.15.8 version
samples/ExportHistoryWebApp/ExportHistoryWebApp.csproj	Updated target framework from net6.0 to net8.0 to align with other samples and support current .NET versions
src/InProcessTestHost/Sidecar/Grpc/TaskHubGrpcServer.cs	Cleaned up exception message by removing TODO prefix and improving punctuation for a more professional user-facing error