@W-11650729 implement nullable type handling in API documentation

[alexpmule]

• Added support for rendering nullable types as "Type or null" in the API documentation.
• Introduced new tests to validate the rendering of nullable types, including scalar and complex types.
• Updated the logic in the PropertyDocumentMixin to correctly identify and handle nullable unions.
• Enhanced the UI to ensure that nullable types do not display union selectors, improving clarity for users.

Improve nullable types rendering in OpenAPI 3.0 specs

🎯 Problem

When a field has nullable: true in OpenAPI 3.0 specs, AMF converts it to a union type (base type + null). The component was rendering all nullable types as "Union" with an "Any of" selector, which created a confusing and verbose UX, especially for simple nullable fields.

This affected all nullable types:

• ✅ Scalars (string, integer, boolean, etc.)
• ✅ Arrays
• ✅ Objects

Example:
A simple timezone field with type: string and nullable: true was showing:

Type: Union
Any of: [TZ] [NULL]  ← Required click to understand it's just "string or null"

💡 Solution

Implemented intelligent nullable detection that simplifies the rendering:

Key Changes:

1. Smart Nullable Detection (_checkNullableUnion)

   • Detects when a UnionShape has exactly 2 members where one is NilShape
   • Works for all types: scalars, arrays, and objects

2. Simplified Rendering

   • Nullable scalars → "String or null", "Integer or null", etc.
   • Nullable arrays → "Array or null" (with Show/Hide to view items)
   • Nullable objects → "Object or null" (with Show/Hide to view properties)
   • Real unions (e.g., string | number) → Keep full Union selector ✅

3. Enhanced Type Detection

   • Updated _computeIsObject() and _computeIsArray() to detect base types in nullable unions
   • Updated _computeIsUnion() to exclude nullable unions from being treated as full unions
   • Complex nullable types properly expand to show their internal structure

Visual Result:

Before:

timezone
  Type: Union           ← Confusing
  Any of: [TZ] [NULL]   ← Extra clicks needed

After:

timezone
  Type: String or null  ← Clear and direct!

For complex types:

metadata                [HIDE]
  Type: Object or null  ← Clear
  
  [When expanded:]
    source: String
    timestamp: DateTime