Fixes dotnet/templating#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`:

```csharp
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.

Commit migrated from dotnet/templating@791a389