Fix FluentValidation not applying to AsParameters type with FromBody

[jeremydmiller]

Summary

Fixes #2358

• Root cause: When [AsParameters] is used with a [FromBody] property, AsParametersBindingFrame overwrites chain.RequestType to the body property type. The FluentValidation policy only checked RequestType, so validators on the [AsParameters] type itself were silently ignored.
• Fix: Added AsParametersType property to HttpChain that tracks the original [AsParameters] type. The HttpChainFluentValidationPolicy now checks for validators on both the RequestType (body type) and the AsParametersType, adding middleware for both when validators exist.
• Documentation: Updated the validation docs with a new "AsParameters with [FromBody]" section (badged 5.25) showing the combined usage pattern.

Files changed

File	Change
HttpChain.cs	Added AsParametersType property
AsParametersBindingFrame.cs	Sets chain.AsParametersType during parameter binding
HttpChainFluentValidationPolicy.cs	Extracted addValidationMiddleware helper; also validates AsParametersType when different from RequestType
FormEndpoints.cs	Added test endpoint with [AsParameters] + [FromBody] + validator
asparameters_binding.cs	Added 3 new tests (happy path, missing Name, missing body)
validation.md	Added documentation section with code sample

Test plan

• 3 new tests pass (happy path, validation failure on AsParameters type, missing body)
• All 9 AsParameters binding tests pass
• All 14 FluentValidation tests pass
• Full HTTP test suite: 540 passed, 0 failed, 10 skipped (pre-existing)
• Existing post_body_services_and_route_arguments test still passes (mentioned in issue as potential regression)

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