Add configure-once ProblemDetails customization for Minimal API

[inghamc]

Addresses #244, and lets consumers resolve #124 and #126 themselves.

ToMinimalApiResult hard-codes the ProblemDetails Title and the "Next error(s) occurred:" Detail prefix, with no Minimal API customization hook (controllers have AddResultConvention; the Minimal API path never got an equivalent — see #244).

This adds an opt-in, configure-once hook:

ResultProblemDetailsOptions.Customize = context =>
{
    context.ProblemDetails.Title = "...";                                      // e.g. localize (#126)
    context.ProblemDetails.Detail = string.Join("; ", context.Result.Errors); // drop the prefix (#124)
};

The hook receives a ResultProblemDetailsContext (result status, HTTP status code, the result, and the ProblemDetails) for every error-class result before it is returned. It defaults to null, so existing output is byte-for-byte unchanged.

Design note: this mirrors ASP.NET Core's CustomizeProblemDetails(Action<context>) idiom, but as a static configure-once hook because the Minimal API conversion runs without an HttpContext / DI. Happy to switch to a per-call overload or a DI-based variant if you'd prefer. Tested on net6.0 / net7.0 / net8.0.

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