feat: add vertex API key support and refactor provider handlers

- Add vertexApiKey field to ProviderSettings for simplified authentication
- Refactor VertexHandler to be fully independent from GeminiHandler
  - Remove inheritance dependency and vertex-specific logic from GeminiHandler
  - Implement standalone VertexHandler with vertex-only authentication methods
  - Add comprehensive debug logging for model selection and API flow
- Update vertex provider UI to include API key input field
- Add i18n support for vertex API key label and placeholder
- Fix vertex tests to use correct default model (claude-sonnet-4@20250514)
- Ensure proper model selection and fallback logic for both providers

Breaking changes:
- VertexHandler no longer inherits from GeminiHandler
- Each provider now has completely separate model sets and authentication

Closes: #TBD

Author: kiwina

packages/types/src/provider-settings.ts

@@ -106,6 +106,7 @@ const bedrockSchema = apiModelIdProviderModelSchema.extend({
 })
 
 const vertexSchema = apiModelIdProviderModelSchema.extend({
+	vertexApiKey: z.string().optional(),
 	vertexKeyFile: z.string().optional(),
 	vertexJsonCredentials: z.string().optional(),
 	vertexProjectId: z.string().optional(),
@@ -290,6 +291,7 @@ export const PROVIDER_SETTINGS_KEYS = keysOf<ProviderSettings>()([
 	"awsBedrockEndpointEnabled",
 	"awsBedrockEndpoint",
 	// Google Vertex
+	"vertexApiKey",
 	"vertexKeyFile",
 	"vertexJsonCredentials",
 	"vertexProjectId",

packages/types/src/providers/vertex.ts

@@ -233,4 +233,5 @@ export const VERTEX_REGIONS = [
 	{ value: "europe-west1", label: "europe-west1" },
 	{ value: "europe-west4", label: "europe-west4" },
 	{ value: "asia-southeast1", label: "asia-southeast1" },
+	{ value: "global", label: "global" },
 ]