[DRAFT] ESQL: Add EMBED_TEXT function for dense vector embeddings

[afoucret]

Summary

Implements the TEXT_EMBEDDING function for ES|QL to generate dense vector embeddings from text using inference model.

Function Signature: TEXT_EMBEDDING(text: string, inference_id: string) -> dense_vector

Example Usage:

FROM documents 
| WHERE KNN(embedding_field, TEXT_EMBEDDING(content, "my-embedding-deployment"), 10)

Fixes #131022

Implementation Status

✅ Completed in this PR:

• TEXT_EMBEDDING_FUNCTION capability with snapshot build gating

• Core Function Infrastructure

  • TextEmbedding function class with proper type validation and serialization
  • InferenceFunction interface for inference-based functions
  • Function registration in EsqlFunctionRegistry

• Analysis of the inference function (validate existence and type of the inference endpoint)

  • Refactored pre-analysis, so it is able to collect inference ids form both Inference plans and inference function
  • Added validation for inference function in the analysis

• Documentation generated from the annotations

🚧 TODO (Before Merge):

• Add a pre-optimizer async phase to the ES|QL query execution
• Execute the inference in the pre-optimizer
• Integration tests and end-to-end validation

Notes

The function is enabled only in snapshot builds.
TEXT_EMBEDDING function is tracked into #131022

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🔍 Preview links for changed docs

• docs/reference/query-languages/esql/kibana/docs/functions/embed_text.md