Optimize MistralAiEmbeddingModel dimensions method

- Calculate and cache values for unknown models only if necessary
- Make known embedding dimensions a mutable map attribute
- Verify the cache mechanism with MistralAiEmbeddingModelTests

Signed-off-by: Nicolas Krier <7557886+nicolaskrier@users.noreply.github.com>

Author: nicolaskrier

models/spring-ai-mistral-ai/src/test/java/org/springframework/ai/mistralai/MistralAiEmbeddingModelTests.java

@@ -29,6 +29,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 /**
@@ -88,9 +89,15 @@ void testDimensionsFallbackForUnknownModel() {
 			.retryTemplate(RetryUtils.DEFAULT_RETRY_TEMPLATE)
 			.build();
 
-		// Should fall back to super.dimensions() which detects dimensions from the API
-		// response
+		// For the first call, it should fall back to super.dimensions() which detects
+		// dimensions from the API response
 		assertThat(model.dimensions()).isEqualTo(512);
+
+		// For the second call, it should use the cache mechanism.
+		assertThat(model.dimensions()).isEqualTo(512);
+
+		// Verify that super.dimensions() has been called once.
+		verify(mockApi).embeddings(any());
 	}
 
 	@Test