deps(nuget): Bump FluentValidation from 11.9.0 to 12.1.1 #551

Job	Run time
Dependency Review	4s
Workflow Lint	31s
OpenAPI Guardrail / OpenAPI Guardrail	38s
Dependency Security Signals	-1s
Demo Director Smoke	-1s
E2E Smoke	0s
Backend Solution Regression	-1s
Load and Concurrency Harness	0s
	1m 10s

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