Add lightweight simple validation with string messages

[jeremydmiller]

Summary

• Add support for Validate/ValidateAsync methods returning IEnumerable<string>, string[], Task<string[]>, or ValueTask<string[]> as a lower-ceremony validation option
• For message handlers: log validation messages as warnings via ILogger and abort processing
• For HTTP endpoints: return a ProblemDetails response with status 400 containing the validation errors
• Register SimpleValidationContinuationPolicy as a default IContinuationStrategy alongside existing HandlerContinuationPolicy
• Add CreateSimpleValidationFrame method to IChain with default handler implementation in Chain<> and HTTP-specific override in HttpChain
• Add documentation section covering all validation options in Wolverine

Test plan

• 8 CoreTests covering all return type permutations (IEnumerable, string[], Task<string[]>, ValueTask<string[]>) with happy and sad paths for message handlers
• 8 Wolverine.Http.Tests covering all return type permutations with happy and sad paths for HTTP endpoints
• Verified existing compound_handlers and problem_details tests still pass

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