feat(models): add step 3.5 flash model

[guozixu2001]

📝 Change Type

Please select the type of change this PR introduces (choose one or more):

• feat: New feature.

💡 Description

This PR introduces support for Step 3.5 flash.

🔗 Related Issues

List any issues this PR closes or relates to:

• Closes [Feature]: Add Step 3.5 Flash #430

✅ Checklist

Please ensure the following points are addressed before merging:

• I have performed a self-review of my own code.
• I have added/updated tests that prove my fix or feature works (if applicable).
• I have updated the documentation (if necessary).
• My code follows the project's style guidelines.