Fix duplicate key handling

[NikolaMilosavljevic]

Fixes #10025

Problem

After migrating from Newtonsoft.Json to System.Text.Json, loading template.json files with duplicate case-insensitive property keys (e.g. "Empty" and "empty") throws an ArgumentException:

System.ArgumentException: An item with the same key has already been added. Key: empty (Parameter 'key')
   at System.Collections.Generic.OrderedDictionary`2.Add(TKey key, TValue value)
   at System.Text.Json.Nodes.JsonObject.InitializeDictionary()
   at System.Text.Json.Nodes.JsonObject.get_Count()
   ...
   at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ConfigModel.TemplateConfigModel..ctor(...)

This affected MAUI templates (maui-blazor, maui-blazor-solution) which intentionally had both "Empty" (PascalCase backward-compat alias) and "empty" (lowercase) symbol definitions.

Root Cause

In src/Shared/JExtensions.cs, the NodeOptions field was configured with PropertyNameCaseInsensitive = true:

private static readonly JsonNodeOptions NodeOptions = new() { PropertyNameCaseInsensitive = true };

This caused JsonObject to use a case-insensitive internal dictionary. When any operation triggered JsonObject.InitializeDictionary() (e.g. accessing .Count, iterating via .ToList()), it attempted to insert both "Empty" and "empty" into the same case-insensitive dictionary, throwing on the duplicate.

This setting was redundant — all case-insensitive property lookups in the codebase already go through the GetPropertyCaseInsensitive() helper method, which manually performs a case-insensitive fallback search.

Fix

File changed: src/Shared/JExtensions.cs

• Removed the NodeOptions field entirely (it only existed to set PropertyNameCaseInsensitive = true)
• Replaced all 6 usages of NodeOptions in JsonNode.Parse() calls with null (uses default JsonNodeOptions, which is case-sensitive)

This means JsonObject now stores properties with their original casing and uses a case-sensitive internal dictionary — which tolerates keys like "Empty" and "empty" coexisting. Case-insensitive lookups continue to work through the existing GetPropertyCaseInsensitive() helper.

Test Added

File: test/Microsoft.TemplateEngine.Orchestrator.RunnableProjects.UnitTests/TemplateConfigTests/GenericTests.cs

Added CanReadTemplateWithDuplicateCaseInsensitiveSymbolKeys — a regression test that verifies a template.json with symbols "Empty" and "empty" loads without throwing.