fix: ensure "async " only appears once in async function signatures

[peterbygrave]

📝 Summary

This PR ensures only a single "async" appears in rendered function signatures.
Previously it was being added by both _get_signature and it's caller in the inspect class.

🔍 Description of Changes

Before:

After:

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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Project	Deployment	Preview	Comments	Updated (UTC)
marimo-docs	Ready	Preview	Comment	Nov 5, 2025 10:25pm

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[peterbygrave]

I have read the CLA Document and I hereby sign the CLA

[mscolnick]

thank you!