🪲 Bug fixes

Fix issues when using the legacy settings indexer

By @dureuill and @ManyTheFish in #6331

• When using the experimental feature "multimodal", removing a fragment would cause an internal error in subsequent search requests
• When using the experimental feature "multimodal", modifying fragments would sometimes cause an internal error at indexing time
• regenerate: false would be ignored when modifying embedder settings
• When a nested field was declared as searchable, it would not be indexed if its parents were not declared as searchable

🔩 Miscellaneous

• Add new CI run that tests the stable settings indexer by @dureuill in #6331