fix: correct CLAUDE_CODE_USE_VERTEX check to match documentation

According to Claude Code docs, the env variable should be checked for value '1', not 'true'
Reference: https://docs.anthropic.com/en/docs/claude-code/google-vertex-ai#4-configure-claude-code

Author: daniel-lxs

src/api/providers/claude-code.ts

@@ -26,7 +26,7 @@ export class ClaudeCodeHandler extends BaseProvider implements ApiHandler {
 		// Filter out image blocks since Claude Code doesn't support them
 		const filteredMessages = filterMessagesForClaudeCode(messages)
 
-		const useVertex = process.env.CLAUDE_CODE_USE_VERTEX?.toLowerCase() === "true"
+		const useVertex = process.env.CLAUDE_CODE_USE_VERTEX === "1"
 		const model = this.getModel()
 
 		// Validate that the model ID is a valid ClaudeCodeModelId