Address code review feedback for large payload tests

[Copilot]

Addresses code review feedback on the large payload validation tests PR to fix build warnings, improve parallelization behavior, and align with project conventions.

Changes:

• Security: Replace SHA1 with SHA256 to avoid CA5350 analyzer warnings with TreatWarningsAsErrors
• Parallelization: Add [DoNotParallelize] to prevent OOM/timeout when 500MB+ tests run concurrently under 4-worker parallel execution
• Timeouts: Add [Timeout] attributes to all test methods (5min for ≤100MB, 10min for ≥500MB)
• Code cleanup:
  • Fix namespace to tests.Tests matching other test files
  • Make RandomStream internal sealed per test assembly conventions
  • Remove unused imports (System.Text, Sisk.Cadente)
  • Use ListeningPort.GetRandomPort() instead of custom TCP listener logic
  • Remove redundant Sisk.Cadente project reference (available transitively)

// Before
namespace tests;
public class RandomStream : Stream { }

[TestMethod]
public async Task TestPayload_10MB() { }

// After  
namespace tests.Tests;
internal sealed class RandomStream : Stream { }

[TestMethod]
[Timeout(300000)]
public async Task TestPayload_10MB() { }

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