Replace custom ValidationBuilder with FluentValidation

[antosubash]

Summary

Replace the custom framework/SimpleModule.Core/Validation/ValidationBuilder.cs (140 LOC) + ValidationResult.cs with FluentValidation 12.1.1. The custom fluent builder reimplemented a narrow subset of what FluentValidation has provided for over a decade.

• −178 LOC net (791 deletions, 613 insertions across 56 files — 12 of the deletions are framework-test files for code that no longer exists)
• 17 validators migrated from static Validate(request) methods → AbstractValidator<T> classes
• 20 endpoints updated to inject IValidator<TRequest> and call ValidateAsync
• 7 modules register their validators via services.AddValidatorsFromAssemblyContaining<ThisModule>()
• Zero response-shape change — GlobalExceptionHandler already emitted RFC 7807 ProblemDetails with an errors extension, which matches FluentValidation's ToDictionary() output

Migration map

Old	New
public static class XxxValidator with Validate(req)	public sealed class XxxValidator : AbstractValidator<T>
new ValidationBuilder().AddErrorIf(...).Build()	RuleFor(x => x.Prop).NotEmpty()...
ValidationResult.IsValid / .Errors	FluentValidation.Results.ValidationResult.IsValid / .Errors
throw new ValidationException(validation.Errors)	throw new Core.Exceptions.ValidationException(validation.ToValidationErrors())

Added ValidationResultExtensions.ToValidationErrors() as a shared helper to bridge FluentValidation's ValidationResult → the Dictionary<string, string[]> shape that ValidationException and the RFC 7807 writer consume.

Kept the existing ValidationException

SimpleModule.Core.Exceptions.ValidationException is the general-purpose "validation failed" exception thrown from many places outside endpoint validators (Chat, UserService, EmailService, PageBuilderService). Removing it would have broadened the PR beyond validation-layer concerns. FluentValidation's own ValidationException is disambiguated by fully qualifying as Core.Exceptions.ValidationException in the 20 migrated endpoints.

Notable edge cases

• UpdateDefaultMapRequestValidator took a runtime parameter (int maxLayers from IOptions<MapModuleOptions>). FluentValidation handles this via constructor injection — the registered validator receives IOptions<MapModuleOptions> and reads maxLayers once at construction.
• Orders' RuleForEach on Items was rewritten as a Custom rule to preserve the aggregated "Items" error-field shape (FluentValidation's RuleForEach would produce keys like "Items[0].Quantity").
• PageBuilder had inline validation in CreateEndpoint.cs / UpdateEndpoint.cs; extracted into CreatePageRequestValidator.cs / UpdatePageRequestValidator.cs.
• Shared email regex in the old SendEmailRequestValidator was removed — FluentValidation's built-in .EmailAddress() rule replaces it across all three email validators.

Documentation updated

• .claude/skills/simplemodule/references/endpoints.md — validation section rewritten
• .claude/skills/minimal-api/references/patterns.md — validation section rewritten
• cli/SimpleModule.Cli/Templates/FeatureTemplates.cs — FallbackValidator template and in-line body replacement now emit AbstractValidator<T>

Historical references in docs/plans/* and docs/specs/* were left unchanged as records of past decisions.

Test plan

• dotnet build — 0 errors, 21 node-modules warnings (unrelated, pre-existing)
• dotnet test — 1195 passed, 0 failed, 1 skipped (pre-existing). 12 fewer than main because ValidationBuilderTests (10) and ValidationResultTests (2) were deleted along with the types they tested.
• No code references to ValidationBuilder, SimpleModule.Core.Validation.ValidationResult, or the old static *Validator.Validate(request) call pattern anywhere in the codebase

Part of a series

PR 2 of 3 replacing custom framework code with trusted OSS libraries:

1. Custom cache → FusionCache (merged: Replace custom ICacheStore with FusionCache #111)
2. [this PR] Custom ValidationBuilder → FluentValidation
3. Custom IEventBus → Wolverine (pending)

Based on latest main (includes #111).

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