fix(core): Handle Umbraco slider {from,to} shape in double config fields

[mattbrailsford]

Summary

Fixes #146 — saving an LLM Safety Judge guardrail rule produces "The JSON value could not be converted to System.Double. Path: $.safetyThreshold" when the guardrail is later evaluated.

Root cause: Umb.PropertyEditorUi.Slider serialises its value as { "from": n, "to": n }, not a plain number. The C# property SafetyThreshold is typed as double, so System.Text.Json fails on deserialization. The Contains and Regex evaluators don't hit this because they bind to text editors. The same latent bug exists on LLMJudgeGraderConfig.PassThreshold.

Fix:

• Added SliderDoubleJsonConverter that accepts a JSON number, a numeric string, or the slider {from, to} object (extracting from).
• Applied via [JsonConverter(...)] to SafetyThreshold and PassThreshold.
• Corrected the slider's EditorConfig aliases — the property editor reads minVal/maxVal, not minValue/maxValue, so the slider previously fell back to a 0–100 range instead of 0–1. Added initVal1=0.7 so the slider initialises at the C# default.

Test plan

• New unit tests in SliderDoubleJsonConverterTests (7 cases: number / string-number / slider object / range / missing from / non-numeric / write)
• Existing guardrail + grader + editable-model tests still green (72 tests)
• Manual verification: create LLM Safety Judge guardrail, attach to Backoffice Copilot agent, send a message — no deserialization error

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