Update TestSparseInferenceServiceExtension to not support text embeddings

[Mikep86]

Does what the title says. Fixes incorrect behavior introduced by #123044, which is wrong for a couple reasons:

• A sparse embedding inference service should not support text embeddings
• The text embedding generation implemented creates a random embedding for each call, which effectively makes it useless for most of our test cases. Many of our test cases rely on consistent result ordering, which won't happen if the embedding changes on subsequent calls with the same input. See generateEmbedding for how we generate a text embedding fit for purpose.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[kderusso]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x