fix: unicode issues with bt sync pull

[Cedric / ViaDézo1er (viadezo1er)]

Fix the thread panic with bt sync pull --window 1é:

bt sync pull --window 1é
✔ Select project · .....
✔ Select object · .....
thread 'main' (233982) panicked at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside 'é' (bytes 1..3) of `1é`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Part of #165

[github-actions]

Latest downloadable build artifacts for this PR commit 3cd9bc2403fa:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/25346209988
• Download all artifacts (GitHub CLI): gh run download 25346209988 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

• ``artifacts-build-global
• ``artifacts-build-local-x86_64-apple-darwin
• ``artifacts-build-local-x86_64-pc-windows-msvc
• ``artifacts-build-local-aarch64-pc-windows-msvc
• ``artifacts-build-local-x86_64-unknown-linux-musl
• ``artifacts-build-local-x86_64-unknown-linux-gnu
• ``artifacts-build-local-aarch64-apple-darwin
• ``artifacts-build-local-aarch64-unknown-linux-gnu
• ``artifacts-plan-dist-manifest
• ``cargo-dist-cache