Analyzer warnings for static handler classes

[danielmarbach]

This pull request introduces a new analyzer diagnostic to prevent static classes from being used as handler classes in NServiceBus, ensuring that only non-static classes are allowed for handler registration via the [Handler] attribute or convention-based mechanisms. The changes add enforcement in both the analyzer logic and the code generation pipeline, as well as new tests to verify the behavior.

Analyzer and Diagnostic Improvements:

• Added a new diagnostic ID NSB0038 (HandlerClassCannotBeStatic) to identify and report static handler classes. [1] [2]
• Updated HandlerAttributeAnalyzer to report the new diagnostic when a static class is decorated with the [Handler] attribute or detected as a convention-based handler. [1] [2] [3]

Code Generation and Parsing Adjustments:

• Modified the handler registration generator to exclude static classes from being considered for handler generation.
• Updated the handler parser to skip static classes during interception logic.

Testing Enhancements:

• Added and updated tests to assert that static handler classes are correctly reported with the new diagnostic, ensuring coverage for both attribute-based and convention-based handlers. [1] [2]