Add context runtime-config updates and decouple label extractor

[Tooyosi]

Summary

This PR does two related things:

1. adds a validated PATCH /contexts/:id update path for context fields and batch runtime metadata
2. decouples label extractor resolution from Galaxy Zoo by introducing a registry that supports both code and DB extractors

What Changed

Context update and batch runtime config

• adds update to resources :contexts
• adds request validation for editable context fields
• merges incoming metadata
• validates metadata.batch through Batch::RuntimeConfig
• rejects unresolved module_name / extractor_name pairs before persisting a context
• adds specs covering valid and invalid payloads

Hybrid label extractor support

• adds LabelExtractorDefinition with JSONB-backed extractor config
• adds LabelExtractors::Registry for shared extractor resolution
• adds LabelExtractors::ConfigurableExtractor for DB-backed task/answer mappings
• updates LabelExtractors::Finder to resolve through the registry instead of the Galaxy Zoo-specific regex
• updates training export header resolution to use the registry
• preserves existing Galaxy Zoo extractor behavior as code-backed registrations

Why

Previously:

• context updates for batch runtime config were not part of the standard contexts resource flow
• label extractor lookup was structurally tied to Galaxy Zoo naming and Ruby classes

This change makes:

• context runtime config editable through the API with validation
• simple new modules onboardable without adding a new Ruby extractor class
• reduction ingestion and training export use the same extractor resolution path