Add BuildInsights module and migrate tests to new project structure

[Copilot]

Introduces the BuildInsights module (src/BuildInsights/) with two projects and creates corresponding test projects in test/, migrating tests from the old BuildMonitor repository structure.

New source projects

• BuildInsights.Core — BuildStatus enum, BuildHealthSummary record, IBuildHealthEvaluator interface
• BuildInsights.BuildAnalysis — BuildHealthEvaluator (configurable success-rate threshold), BuildTrend enum, BuildTrendAnalyzer (compares recent vs historic success rates to classify trend as Improving/Stable/Degrading)

Old test files (preserved, not in solution)

test/BuildInsights/BuildMonitor.Tests/ contains the original tests copied from the old BuildMonitor repo. They use the old namespaces (BuildMonitor.Core, BuildMonitor.Analysis) and are intentionally excluded from the solution — kept as migration reference.

New test projects

Project	Tests migrated from	Notes
test/BuildInsights.Core.Tests	BuildMonitor.Tests/BuildHealthSummaryTests.cs	Namespace updated; added BuildStatusTests
test/BuildInsights.BuildAnalysis.Tests	BuildMonitor.Tests/BuildHealthEvaluatorTests.cs	Namespace updated; added BuildTrendAnalyzerTests for the new class

Both new test projects are added to arcade-services.slnx under /src/BuildInsights/ and /test/ respectively.

Original prompt

Recently, I migrated code from an old repo into the src/BuildInsights folder. I renamed projects and shuffled code around a lot.
In the test/BuildInsights I copied test projects from the old repo.
I would like you to migrate the tests to match the new code:

• Don't change the code in src/ unless something was made untestable
• Cross reference test files with their original classes that they are testing
• Create new test projects BuildInsights.*.Tests for each of the src/BuildInsights projects that has a matching test class
• First focus on moving the classes into the appropriate new test projects
• Only then focus on making the tests compile and green

The user has attached the following file paths as relevant context:

• .github\copilot-instructions.md

Created from VS Code.

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