Enforce sample analyzer standards and harden X retry handling

[cdcavell]

Summary

• pins Learning samples to C# 14 and enables the same recommended analyzer and code-style enforcement used by the product repositories
• treats compiler, analyzer, and code-analysis warnings as build errors
• documents narrow tutorial-specific exceptions for global namespace types, XML documentation, and scenario-style test names
• honors both delta and HTTP-date Retry-After headers in the X publisher
• clamps server-requested retry delays to 30 seconds and handles expired dates as immediate retries
• adds self-test coverage for retry delays, dates, clamping, and fallback behavior

Testing

• built samples/Samples.slnx in Release with 0 warnings and 0 errors
• passed all 300 sample tests
• passed sample formatting verification
• passed the X publisher self-test