docs(generativeai): add region tags for code samples

[msampathkumar]

Description

Adding generativeaionvertexai_ region tags to replace old aiplatform_

In near future GenAI code samples, will only use generativeaionvertexai_ region tag prefix.

Fixes: b/350918509

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• [ ] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] Tests pass: mvn clean verify required
• [ ] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 10 region tags.

- [aiplatform/src/main/java/aiplatform/CreatePipelineJobModelTuningSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/CreatePipelineJobModelTuningSample.java#L20), tag `generativeaionvertexai_sdk_tuning` - [aiplatform/src/main/java/aiplatform/EmbeddingModelTuningSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/EmbeddingModelTuningSample.java#L20), tag `generativeaionvertexai_sdk_embedding_model_tuning` - [aiplatform/src/main/java/aiplatform/PredictChatPromptSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictChatPromptSample.java#L20), tag `generativeaionvertexai_sdk_chat` - [aiplatform/src/main/java/aiplatform/PredictCodeChatSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictCodeChatSample.java#L20), tag `generativeaionvertexai_sdk_code_chat` - [aiplatform/src/main/java/aiplatform/PredictCodeCompletionCommentSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictCodeCompletionCommentSample.java#L20), tag `generativeaionvertexai_sdk_code_completion_comment` - [aiplatform/src/main/java/aiplatform/PredictCodeGenerationFunctionSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictCodeGenerationFunctionSample.java#L20), tag `generativeaionvertexai_sdk_code_generation_function` - [aiplatform/src/main/java/aiplatform/PredictImageFromImageAndTextSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictImageFromImageAndTextSample.java#L20), tag `generativeaionvertexai_sdk_text_image_embedding` - [aiplatform/src/main/java/aiplatform/PredictTextEmbeddingsSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictTextEmbeddingsSample.java#L20), tag `generativeaionvertexai_sdk_embedding` - [aiplatform/src/main/java/aiplatform/PredictTextPromptSample.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/aiplatform/src/main/java/aiplatform/PredictTextPromptSample.java#L20), tag `generativeaionvertexai_sdk_ideation` - [vertexai/snippets/src/main/java/vertexai/gemini/GetTokenCount.java:20](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/88d58c9ac5f70f885fa5639acf38a174cad31fbd/vertexai/snippets/src/main/java/vertexai/gemini/GetTokenCount.java#L20), tag `generativeaionvertexai_gemini_token_count`

You are about to delete 1 region tag.

- [vertexai/snippets/src/main/java/vertexai/gemini/GetTokenCount.java:19](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/300f3b9bd9b03d6003661e1445c37a836a2c5f1a/vertexai/snippets/src/main/java/vertexai/gemini/GetTokenCount.java#L19), tag `generativeaionvertexai_gemini_token_count`

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