chore: add env templates to more dashscope examples

[however-yir]

Summary

• add .env.example files to a few more DashScope-backed example modules

Why

Some frequently used examples still require users to infer the needed environment variable from the README or application.yml. A tiny template makes local setup more obvious and continues the same low-risk contribution pattern as the existing env-template additions.

Scope

• add .env.example to spring-ai-alibaba-helloworld
• add .env.example to spring-ai-alibaba-chat-example/dashscope-chat
• add .env.example to spring-ai-alibaba-chat-example/qwq-chat
• add .env.example to spring-ai-alibaba-image-example/dashscope-image

Testing

• not run (template-only change)

[however-yir]

Hi team, checking in on this chore/docs PR that adds env templates to more DashScope examples. Goal is to make first-run setup more consistent across examples. Happy to adjust file placement/naming if needed.