NestedPlaceholderPart class

A PlaceholderPart that represents a nested placeholder expression with optional fallback.

This is used to represent placeholders whose keys or fallbacks themselves contain inner placeholder expressions, such as:

#{#{env:dev.name}:#{default.value}}

In this example:

• keyParts would resolve #{env:dev.name}
• defaultParts would resolve #{default.value}

The resolution process:

1. Resolves keyParts to get the final key.
2. Tries to resolve that key using resolveRecursively.
3. If unresolved, tries defaultParts if provided.
4. If still unresolved, delegates to PlaceholderPartResolutionContext.handleUnresolvablePlaceholder.

This allows for recursive resolution and fallback substitution, enabling powerful and composable placeholder configurations.