Fix #668 STJ JsonElement unwrap, #676 docs cleanup, regression guard for #692

[YogeshPraj]

Summary

Batch-5 covers four of the six remaining issues. The other two (#679 rule-chaining via @RuleName, #710/#709 AGV spam) are not addressed in this PR — see "Not in this PR" below.

Fixes #668 — System.Text.Json migration: ExpandoObject + JsonElement comparisons broken

After #599 swapped Newtonsoft for STJ, properties on an ExpandoObject built from JSON came through as JsonElement values. Rule expressions like input1.country == "india" then failed with:

The binary operator Equal is not defined for the types 'System.Text.Json.JsonElement' and 'System.String'.

Utils.CreateAbstractClassType now infers the native CLR type from a JsonElement.ValueKind (string / int / long / double / bool / null), and Utils.CreateObject unwraps JsonElement scalars to their native values before assigning them. Nested objects and arrays inside the JsonElement keep their JsonElement shape — that path is for typed Newtonsoft-style models that weren't using ExpandoObject anyway.

Fixes #676 — ErrorType removed from docs examples

ErrorType was removed from the Rule model in 4.0.0 but still appeared in JSON examples across README.md, docs/Getting-Started.md, and docs/index.md. Removed.

Regression guard for #692 — nullable DateTime comparisons against null

User reported "null < DateTime returns false in both directions after 5.0.4." That's actually standard C# Nullable<T> semantics — both null < x and null > x are false. The pre-5.0.4 behavior was a Newtonsoft / Dynamic.Core quirk, not the documented contract.

Added an explicit test documenting both the current behavior and the canonical workaround:

"!input1.Dt.HasValue || input1.Dt < someDate"