Add Vertex AI express mode support (#3977)

* Add Vertex AI express mode support
Split Google AI Studio and Vertex AI

* Add support for Vertex AI, including updating default models and related settings, modifying frontend HTML to include Vertex AI options, and adjusting request processing logic in the backend API.

* Log API name in the console

* Merge sysprompt toggles back

* Use Gemma tokenizers for Vertex and LearnLM

* AI Studio parity updates

* Add link to express mode doc. Also technically it's not a form

* Split title

* Use array includes

* Add support for Google Vertex AI in image captioning feature

* Specify caption API name, add to compression list

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Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

public/scripts/reasoning.js

@@ -114,6 +114,7 @@ export function extractReasoningFromData(data, {
                 case chat_completion_sources.OPENROUTER:
                     return data?.choices?.[0]?.message?.reasoning ?? '';
                 case chat_completion_sources.MAKERSUITE:
+                case chat_completion_sources.VERTEXAI:
                     return data?.responseContent?.parts?.filter(part => part.thought)?.map(part => part.text)?.join('\n\n') ?? '';
                 case chat_completion_sources.CLAUDE:
                     return data?.content?.find(part => part.type === 'thinking')?.thinking ?? '';