Add RequirementResult as validation return type for Validate methods

[jeremydmiller]

Summary

• Adds RequirementResultContinuationPolicy — a new IContinuationStrategy that detects Validate/ValidateAsync methods returning RequirementResult
• When Branch == Continue, processing continues normally
• When Branch == Stop, message handlers log each message as a warning and abort; HTTP endpoints return ProblemDetails with 400 status
• If Messages are empty on stop, handlers log "Invalid Request" and HTTP endpoints set ProblemDetails.Detail = "Invalid Request"
• Separate Frame classes for reuse: RequirementResultHandlerFrame (message handlers) and RequirementResultHttpFrame (HTTP endpoints)
• Follows the same pattern established in PR Add lightweight simple validation with string messages #2272 for IEnumerable<string> simple validation

Test plan

• 5 message handler tests (sync, async, empty messages — happy and sad paths)
• 7 HTTP endpoint tests (sync, async, empty messages — happy and sad paths, Detail assertion)
• All 532 existing HTTP tests still pass

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