mistralai[minor]: Add embeddings (#15282)

- **Description:** Adds MistralAIEmbeddings class for embeddings, using
the new official API.
- **Dependencies:** mistralai
- **Tag maintainer**: @efriis, @hwchase17
- **Twitter handle:** @LMS_David_RS

Create `integrations/text_embedding/mistralai.ipynb`: an example
notebook for MistralAIEmbeddings class
Modify `embeddings/__init__.py`: Import the class
Create `embeddings/mistralai.py`: The embedding class
Create `integration_tests/embeddings/test_mistralai.py`: The test file.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/partners/mistralai/tests/integration_tests/test_embeddings.py

@@ -0,0 +1,19 @@
+"""Test MistralAI Embedding"""
+from langchain_mistralai import MistralAIEmbeddings
+
+
+def test_mistralai_embedding_documents() -> None:
+    """Test MistralAI embeddings for documents."""
+    documents = ["foo bar", "test document"]
+    embedding = MistralAIEmbeddings()
+    output = embedding.embed_documents(documents)
+    assert len(output) == 2
+    assert len(output[0]) == 1024
+
+
+def test_mistralai_embedding_query() -> None:
+    """Test MistralAI embeddings for query."""
+    document = "foo bar"
+    embedding = MistralAIEmbeddings()
+    output = embedding.embed_query(document)
+    assert len(output) == 1024