Add TranslationManagerInterface programmatic admin management API

[tito10047]

Summary

This PR introduces a public service interface (TranslationManagerInterface) that allows
client applications to build admin UIs for managing translations without depending on
internal bundle classes or CLI commands.

Instance-level operations (read, write, delete, status, cache) accept the entity object
directly. Class-level operations (field discovery, listing objects by type, bulk delete,
type-level cache) accept ClassName::class ? a FQCN string.

Previously the only way to manage translations was via object-translation:export /
object-translation:import CLI commands. This PR opens up the same capabilities
programmatically through a stable, injectable service.

Motivation

Applications built on this bundle often need an admin panel where editors can:

• See which entities have translations and which fields are translatable
• View translation completeness per entity instance and locale
• Create or update individual field translations inline
• Delete stale or incorrect translations
• Invalidate stale cache entries after making changes

None of this was possible without reaching into @internal classes.