fix: avoid utf8 panic in file preview

[Bortlesboat]

Fixes PlatformNetwork/bounty-challenge#53262.

What changed:

• app-server write_file metadata preview now truncates by Unicode scalar count instead of slicing at byte index 500.
• Added a regression test with a multi-byte character crossing the old byte boundary.

Verification:

• cargo test -p cortex-app-server write_file_content_preview_truncates_on_utf8_boundary_without_panic -- --nocapture
• cargo test -p cortex-app-server --lib -- --skip tools::shell::tests::test_execute_shell
• cargo check -p cortex-app-server
• cargo fmt --check --package cortex-app-server
• git diff --check

Note: the skipped app-server lib test is the existing Windows-only shell test that expects echo as an executable.

[greptile-apps]

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