Return ProblemDetails with error detail on every failure arm and compose ProblemDetails customization

[Vulthil]

Summary

• ProblemHttpResult/ObjectResult now carry the error's code+description on every failure arm on both
  public paths (typed IResult via ToIResult/CustomResults.Problem, and MVC via ToActionResult) — NotFound
  and Conflict no longer return bare, detail-less responses.
• ErrorType.Problem now maps to HTTP 400 (client-addressable) instead of being lumped into the 500 default
  arm, on both paths.
• AddProblemDetailsHandling now composes the package's requestId/traceId enrichment with any
  consumer-supplied CustomizeProblemDetails via PostConfigure, instead of one silently overwriting the other.
• The OpenAPI 500-response transformer now always merges (preserving an existing response's Description
  instead of discarding it on net10 with nullable Content) and attaches a ProblemDetails schema to the
  problem+json media type, on both net9 and net10.
• MVC ToActionResult no longer populates ModelState for non-validation errors that never read it.
• BaseController.Logger is now cached after first resolution instead of re-resolving ILoggerFactory +
  creating a logger on every access.
• AddEndpoints skips open-generic IEndpoint implementors (previously registered them, which blew up later
  at resolution); MapEndpoints resolves endpoints from a dedicated scope instead of the root provider.
• Aligned ArgumentNullException.ThrowIfNull guards across the public extension methods in these files.

Observable HTTP payload changes

• 404/409 responses on both public paths now return an RFC 7807 body instead of an empty body.
• Failures with ErrorType.Problem now return HTTP 400 instead of 500.
• The generated OpenAPI document's synthesized 500 response now has a real ProblemDetails schema; an
  existing 500 response's Description is preserved when merged instead of being replaced.

Verification

• Full solution build: clean, 0 warnings, 0 errors.
• New Vulthil.SharedKernel.Api.Tests project (31 tests): one test per ErrorType arm on both paths,
  the CustomizeProblemDetails composition test, transformer merge/schema on both net9.0/net10.0, and
  endpoint-registration hardening. All 31 pass on the fix; 21 of 31 fail against unfixed main (verified by
  stashing the src changes and re-running), confirming each is load-bearing.
• WebApi sample's own test suite (Testcontainers-backed): 8/8 pass — sample still boots, happy path unaffected.
• Manually exercised a live 404 through the running sample and confirmed the new RFC 7807 payload shape.
• PublicAPI.Shipped.txt/PublicAPI.Unshipped.txt untouched — no public signature changes.
• Known follow-up (out of scope, owned by the parallel Results-semantics session):
  docs/articles/result-pattern.md's mapping table still says Problem → 500 and needs updating to 400.

Backport to v1.0: no — observable HTTP payload changes are minor-appropriate; ships in 1.1.0.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.