Fix missing using directives in DiagnosticTest.cs

[Copilot]

Description

Restores required using directives to DiagnosticTest.cs that were inadvertently removed in #4078, causing compilation failures across all TFMs (particularly net462, where implicit/global usings are not available).

Restored namespaces:

• System — Environment, Console, Guid, Func
• System.Threading.Tasks — Task<T> for async stdout/stderr reads
• Xunit / Xunit.Abstractions — Assert, ConditionalFact, ITestOutputHelper
• Microsoft.DotNet.RemoteExecutor — RemoteExecutor, RemoteInvokeOptions, RemoteInvokeHandle
• Microsoft.SqlServer.TDS — TDSMessage, TDSMessageCollection, TDSMessageType
• Microsoft.SqlServer.TDS.Done — TDSDoneToken, TDSDoneTokenStatusType, TDSDoneTokenCommandType
• Microsoft.SqlServer.TDS.EndPoint — ITDSServerSession
• Microsoft.SqlServer.TDS.Error — TDSErrorToken
• Microsoft.SqlServer.TDS.SQLBatch — TDSSQLBatchToken
• Microsoft.SqlServer.TDS.Servers — TdsServer, TdsServerArguments, QueryEngine

Issues

Addresses review feedback on #4078: #4078 (comment)

Testing

No logic changes — compilation fix only. The diagnostic tests themselves remain unchanged. Build validation across net462, net8.0, and net9.0 confirms the file compiles correctly with all TFMs.

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
• Code of Conduct
• Best Practices
• Coding Style
• Review Process

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[Copilot]

Pull request overview

Restores missing using directives in the manual diagnostics test file so it compiles again across all target frameworks (including net462, where implicit/global usings aren’t available).

Changes:

• Re-adds System and System.Threading.Tasks usings needed for types like Environment, Func<>, and Task<>.
• Re-adds required test/framework and TDS server usings (Xunit*, Microsoft.DotNet.RemoteExecutor, Microsoft.SqlServer.TDS*) referenced throughout the file.