ActiveContext embedding update: add validations and put inside transaction

What does this MR do and why?

For the ActiveContext Model Selection UI, we need to validate that the model_type and dimension values are supported. Instead of having this validation in a controller, it's simpler to have it on the json schema.

We also need to make sure that the next_indexing_embedding_model is not set if Task chain creation fails, so we don't end up in an incomplete scenario where there is a next_indexing_embedding_model but there are no tasks. See !232069 (comment 3265121637).

In this MR:

• we move the embedding metadata to its own json schema and reference it in the Collection#metadata schema
• update the embedding metadata schema to have an enum of valid values for model_type and dimensions
• update the spec for Ai::ActiveContext::Collection and Ai::ActiveContext::EmbeddingModelActivationService to test invalid values
• In Ai::ActiveContext::EmbeddingModelActivationService#execute, put the metadata update and tasks creation within a transaction so that the metadata change rolls back if there is an error in the tasks creation.

References

• ActiveContext Model Selection UI issue: #582638 (closed)
• ActiveContext Model Selection UI MR: !230649 (merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

The changes in this MR (json schema + wrapping DB updates in a transaction) make use of well-known and established ruby frameworks, so the unit tests should cover the required validations.

MR acceptance checklist

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Related to #582638 (closed)