Add CT0012: require XML summary on Azure Function classes

[magesoe]

Why

The existing CT0006 rule enforces XML <summary> documentation on Dataverse plugin classes. Azure Function classes benefit from the same discipline — without it, there's no at-a-glance description of what a function class does, making the codebase harder to navigate.

What

Adds CT0012 (AzureFunctionDocumentationAnalyzer), a DiagnosticSeverity.Warning rule in the Documentation category. It fires on any class that contains a method decorated with:

• [Function("...")] — isolated worker model (Microsoft.Azure.Functions.Worker)
• [FunctionName("...")] — in-process model (Microsoft.Azure.WebJobs)

…and lacks either a non-empty XML <summary> or an <inheritdoc> tag on the class declaration. The diagnostic message includes the class name for easy identification.

Classes with no Azure Function trigger methods are unaffected, as are classes whose [Function]/[FunctionName] attributes come from an unrelated namespace.

Tests

14 new test cases in AzureFunctionDocumentationAnalyzerTests cover:

• Missing comment, empty summary, whitespace-only summary → trigger
• Valid single-line and multi-line summary → no trigger
• <inheritdoc> with and without cref → no trigger
• <remarks>-only (no <summary>) → trigger
• [FunctionName] (in-process model) → same behaviour
• Multiple function methods on one class → triggers once
• Multiple classes with mixed docs → only undocumented classes trigger
• Attribute from a wrong namespace → no trigger
• Class with no function methods → no trigger

All 146 tests pass (132 pre-existing + 14 new).