build: pin unicode-segmentation for vortex tests

[JingsongLi]

Summary

Main CI is still hanging after reverting #346, so the object-storage change does not appear to be the root cause.

Comparing the successful #350 Ubuntu unit job with the hanging main Ubuntu unit job shows the same compiled crate set except for one resolved patch dependency:

• successful feat: add bucket and level statistics to ManifestFileMeta #350 PR merge-ref: unicode-segmentation v1.13.2
• hanging main run: unicode-segmentation v1.13.3

This PR pins unicode-segmentation to =1.13.2 through the paimon crate so CI resolves back to the dependency set that passed before the Vortex tests started hanging.

Changes

• Add an exact unicode-segmentation = "=1.13.2" dependency in crates/paimon/Cargo.toml.
• Leave the feature: add more object storage backends #346 object-storage changes intact.

Testing

• cargo tree -p paimon --features fulltext,vortex -i unicode-segmentation resolves unicode-segmentation v1.13.2
• cargo test -p paimon arrow::format::vortex::tests::test_vortex_read_with --all-targets --features fulltext,vortex -- --nocapture
• cargo test -p paimon-datafusion --features vortex --test vortex_tables -- --nocapture

Notes

This is intended as a targeted unblock while the interaction between the latest unicode-segmentation patch and the Linux Vortex tests is investigated further.

[leaves12138]

LGTM. This is a small dependency pin, and all CI jobs are green. Pinning unicode-segmentation to the last known-good version for the Vortex test hang is reasonable.