Fix app-server preview truncation on UTF-8 boundaries

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53262.
Fixes PlatformNetwork/bounty-challenge#53271.

Summary

• Add a shared app-server preview helper that truncates at the nearest valid UTF-8 character boundary.
• Use it for filesystem write_file() metadata previews and web fetch_url() response previews.
• Apply the same helper to web_search() result truncation, which had the same byte-index slicing pattern.

Validation

• cargo +1.90.0 fmt --package cortex-app-server --check
• git diff --check
• Isolated rustc +1.90.0 --edition=2024 --test harness covering 500-byte and 100000-byte UTF-8 boundary cases

Note: targeted cargo test -p cortex-app-server preview::tests -- --nocapture is still blocked before running tests on this Windows/MSVC environment by the existing aws-lc-sys C build failure.

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.