fix(adapters): raise a clear error for Groq embeddings

[BrUn3y]

Description

#1537
The GroqEmbeddingModel shipped with a default model_id of "llama-3.1-8b-instant", which is a Groq chat model — not an embedding model. Worse, Groq does not expose a text embeddings API at all (Groq models), so constructing the model without an explicit model_id (e.g. EmbeddingModel.from_name("groq") or GroqEmbeddingModel()) produced a cryptic provider failure on the first request.

This PR makes the limitation explicit instead of failing obscurely:

• GroqEmbeddingModel.__init__ now raises a clear NotImplementedError explaining that Groq has no embeddings API and pointing to providers that do (OpenAI, Ollama, watsonx, Gemini, MistralAI). Because the dynamic model loader resolves the same class, this also covers EmbeddingModel.from_name("groq").
• Removes the broken GroqEmbeddingModel() usage from python/examples/backend/providers/groq.py.
• Updates the supported-providers table in docs/src/content/docs/modules/backend.mdx: Groq embedding ✅ → ❌, and drops the misleading GROQ_EMBEDDING_MODEL env var.
• Adds unit tests (python/tests/adapters/groq/) asserting both construction paths raise the clear error.

No breaking change for working code: there was no valid Groq embedding configuration before, so any prior usage already failed at runtime — now it fails immediately with an actionable message.

Note: the TypeScript GroqEmbeddingModel already requires an explicit modelId (it throws ValueError("Missing modelId!") otherwise), so it never carried the invalid-default bug. Making the TS side consistently reject Groq embeddings can be a separate follow-up.

Checklist

General

• I have read the appropriate contributor guide: Python / TypeScript
• I have signed off on my commit: Python instructions / TypeScript instructions
• Commit messages and PR title follow conventional commits
• Appropriate label(s) added to PR: Python for Python changes, TypeScript for TypeScript changes

Code quality checks

• Code quality checks pass: mise check (mise fix to auto-fix)

Testing

• Unit tests pass: mise test:unit
• E2E tests pass: mise test:e2e
• Tests are included (for bug fixes or new features)

Documentation

• Documentation is updated
• Embedme embeds code examples in docs. To update after edits, run: Python mise docs:fix

[gemini-code-assist]

Code Review

This pull request disables the unsupported Groq embedding model and updates the documentation, examples, and tests accordingly. Since Groq does not offer a text embeddings API, the GroqEmbeddingModel class now explicitly raises a NotImplementedError with a clear, actionable error message. I have no feedback to provide.

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[Tomas2D]

Correctly resolves #1537 — raises a clear NotImplementedError for both direct construction and the from_name path, with matching tests and docs updates. CI green. Thanks @BrUn3y! 🐝