CELDEV-1351 Extract Presentation REST reuse seams

[fpichler]

Summary

• extract reusable inner-content rendering for renderedContent and renderedExtract
• expose typed presentation root metadata while preserving legacy rendering behavior
• isolate rendering context and preserve legacy error behavior
• synchronize with the 7.3 development line
• encode Java whitespace in generated navigation CSS class tokens consistently for token and legacy-string output

Requirements

• CELDEV-1351

Validation

• final NavigationTest: 100 passed
• legacy rendering, CSS-token parity, Unicode-whitespace, and context-isolation tests passed
• 7.3 Maven model and focused core validation passed
• git diff --check

Delivery chain

This is the first PR in the coordinated 7.3 Presentation REST API delivery.