ENH: Add missing return type annotations to utility methods

[Samearth17]

Description

I noticed that several utility methods/functions in the codebase do not currently have explicit return type annotations.

Adding these annotations would improve static analysis, IDE support, and consistency with the existing typed codebase.

The main areas I noticed are:

• src/smolagents/tools.py
• src/smolagents/memory.py
• src/smolagents/vision_web_browser.py

I have investigated these locations and prepared a small implementation that adds the missing return type annotations.

Before proceeding further, I wanted to confirm that this type of code-quality improvement is in scope and would be useful for the project, in accordance with the contribution guidelines.

Related work: #2676

Would a maintainer consider this issue for the status:accepted label if appropriate?

[Samearth17]

Hi maintainers!

I opened this issue after reading the contribution guidelines and noticed that code changes larger than a typo should first have an accepted issue.

I have an existing implementation in PR #2676 covering the files mentioned here. I’d be happy to adjust the scope or implementation based on your feedback.

Would this be appropriate for status:accepted?